The first four bytes of the data section (e.g. bytes 12-15 in the table above) give the starting point of the first Type in the data section.  I.e. if bytes 12-15 give the number 5,000, this would equate to byte 5,012 including the header.

From this point on, the 32-bit numbers will be referred to in the 'correct' order.  I.e. not like the "MIRP" in the above table.  If you're looking at a 3DO file with a hex editor, the 4 bytes for each number will be in the 'incorrect', or network order: b3 b2 b1 b0.

Some numbers are 32bit integers, others are 32bit floats.  The floats are generally used for coordinates (vertex & texture), and can be identified by their huge values if you read them in as an integer :-)

Most of the Types point to other Types.  Those that don't are the T-8XX's which define polygons and a couple of others.  Basically, the PRIM structure can be drawn as a tree.  (Sometimes a recursive tree - track.3do's).

Some more notes:

• Colour means a 4 byte number in the format Alpha Red Green Blue ('correct' order).
• Offsets are offsets within the data section, i.e. not including the header.
• All numbers are given in hexadecimal, without the 0x prefix, but sometimes with, so beware :)
• Some offsets are 0xFFFFFFFF.  As far as I can tell, this means that it should be ignored.
• Also, some vertices/planes, etc. are 0xFFFFFFFF.  These should probably be ignored.
• String offset means an offset into the STRN section, not including the STRN header.
• Vertex numbers are not offsets into the VERT section.  To get the offset into the VERT data section, multiple the Vertex number by 16.
• The same goes for vertex normals within the NORM section (if there is one in the 3do) & planes within the PLAN section.

These images mean: general node; leaf node (primitive or other); car-specific; finished!

General format is T-x (where x is hexadecimal); brief description; list of components that make up the type (nearly all 32bit integers/floats, where entries end with ..., it means there are a variable number of them); Long description

For the quick look table below, these images signify the different components:
=offset (i.e. pointer to a child node)
=colour
=string offset
=scale (float)
=integer
=float
=vertex number
=plane number
=normal number
= texture x,y pairing (2 floats)
number = this number appears
count = there are this many 'things' to follow - where a thing is in brackets and followed by ...

Type (hex)	Format	Description
003	3, , , , ,	SRB Chooser & Placer & Scaler
004	4,count,  's...	Multi child node
005-1	5, ,1,	Selector: mip file without display class
005-4	5, ,4,	Selector: ? display class ?
005-5	5, ,5, ,	Selector: mip file with display class
005-8	5, ,8, , ,	Selector: ?
005-C	5, ,C, , , ,	Selector: ?
005-D	5, ,D, , , ,	Selector: ?
005-11	5, ,11,count, 's...	Selector: ?
005-20	5, ,20,	Selector: ?
005-24	5, ,24, ,	Selector: ?
005-2C	5, ,2C, , , ,	Selector: ?
006	6, ,	Plane, single child node
007	7, , ,	Plane, double child node
008	8, , , , ,	Plane, quad child node
009	9, , , ,	Plane, triple child node
00A	A, , , ,	Plane, triple child node
00B	B, , ,	Plane, double child node
00D	D, (dx), (dy), (dz), , , , ,	Positioner
00E	E, ,0	3DO File Chooser
00F	F, , , , , ,-1, 0,0,0,0, count, ( , , , )'s...	Track visibility box
010	10,count, 's...	?Chooser of some description?
011	11, , , ,count, ( , )'s...	Trk relation?
013	13, (dx),  (dy),  (dz),  ,  ,  ,  ,	Positioner (3DO)
014	14, ,	?
016	16, (dx),  (dy),  (dz),  ,  ,  ,  ,	Positioner
017	17,count, 's...	Room?
019	19, , , , , , , , ,	Bounding cuboid - seen lots of places
01F	1F,0, ,count,  's...,  's...	Car Polygon, textured
020	20,0,0, ,count,  's...,  's...,  's...	Car Polygon, textured, smooth shading
021	21,0, ,count,  's...,  's...,  's...	Car Polygon, textured, smooth shading, normals
815	815, , ,	Line
81B	81B, ,count, 's...	Polygon, flat shading
81C	81C, ,count, 's..., 's...	Polygon, smooth shading
81D	81D, ,count, 's..., 's...	Polygon, normals
81E	81E, ,count, 's..., 's..., 's...	Polygon, smooth shading, normals
81F	81F,0, ,count, 's..., 's...	Polygon, textured
820	820,0, ,count, 's..., 's..., 's...	Polygon, textured, smooth shading
821	821,0, ,count, 's..., 's..., 's...	Polygon, textured, normals
A03	A03, , , , ,	SRB Selector

Number	Type of thing in class	Menu option that affects the class
0	Track	Track Textures
1	Grass, curb, edge, bank	Terrain Textures
2	Fence, armco	Wall Textures
4	(external)Engine	Car Textures
8	Trees?	??
10	Startline, starting grid boxes	?Track Textures?
13	Face, rim, shock, etraymap	?Car Textures?
17	Steering	?Cockpit Texture?
18	???insidelf	??
19	Knees, driver, arms, neck	Driver Textures
20	Racing groove	Racing Groove

General Nodes

T-3  .SRB File

3, vertex number, colour, string offset, float, int

Colour may or may not be a colour.  I've only seen it as 0xffffffff, so it might be a pointer (never used?)
Vertex number suggest the centre of the SRB, i.e. as if it were an object in 3-space.
String offset gives the filename of the .SRB file, when looked up within the String section (string header not included).
The float looks like it defines a scale, and the int seems to be a bit pattern (e.g. 65536, 0...)

4, number of offsets, offset...
This type acts as a simple multi-root node.

5, offset, sub-type, other data....
T-5₁
5, offset, 1, string offset
String offset points into the String section, and is used to choose a MIP file for the textures.
T-5₄
5, offset, 4, number
Only ever seen as the root node.  The number has unknown meaning.
T-5₅
5, offset, 5, String offset, number
String offset points into the String section, and is used to choose a MIP file for the textures.  The last number has an unknown meaning (probably something to do with the graphics options).
T-5₈
5, offset, 8, int1, int2, float
I'd have thought one of the int's specified a string offset but have been unable to determine which.  The float is always(?) 0.3 or 0.6.
T-5_{c(dec 12)}
5, offset, 12, int1, int2, int3, float
See sub-type 8.
T-5_{d(dec 13)}
5, offset, 13, string offset, int1, int2, float
See sub-type 8 (but this time I know the string offset is where it is :-)
T-5_{11(dec 17)}
5, .3do offset, 17, number of .mip string offsets, .mip string offsets...
This looks like it chooses a single .3do file, and several .mip files.  I've seen it used in the lotus wheel .3do's (of which there are many).
Sometimes, the .mip offsets are the same.  I believe this means that two different file extensions are used, e.g. file.mip & file.3do.  This would be a bit bizarre, because usually the .3do that contains this T-5₁₁ has the same filename, so it would be calling itself.  Or it could be that these multiple calls reference the HAND sections of the referenced 3do??
T-5_{1c(dec 28)}
Haven't done this one yet...
T-5_{20(dec 32)}
5, offset, 32, int
The int is a number used to choose a predefined texture, possibly from a list of mips chosen by another node.  This node has only been found in car wheels so far, where the int is used for: 0 or 1 = tire wall texture, 2=tire tread texture.
T-5_{24(dec 36)}
5, offset, 36, int (0), int (4)
Looks suspiciously like sub-type 4, so the first int is probably a string offset, but I can never tell when it's zero :-(
T-5_{2c(dec 44)}
5, offset, 44, int (0), int (4), int (0xA), float (0.3 or 0.6)
Mmm...

Where 'above' is taken from the plane in the direction of X,Y,Z, and below is in the opposite direction.

T-6    Single child with plane

6, plane number, offset
The plane number references the PLAN section.  The offset contains the polygons visible when the viewer is 'above' the plane.

7, plane number, offset, offset
A dual child node, with a plane.  Each offset is followed only if the contents can be seen.

8, plane number, offset1, offset2, offset3, offset4
This behaves like T-9 and T-A, in that the first and last offsets are the sub-trees 'above' and 'beneath' the plane.  The second and third offsets are the sub-trees 'on' the plane, one viewed from above, the other when viewed from beneath.
This node is used in places like a fence bordering the track, where the fence can be viewed from both sides.

9, plane number, offset1, offset2, offset3
The plane number references into the PLAN section.  One or more of the offsets is often NULL (i.e. 0xffffffff).
_{The first offset is for things that can be seen 'below', the second for 'above', and the third is always followed, as far as I can tell (above/below, that is).}

Same as T-9
No idea why T-A and T-9 are the same, so there must be some difference.  Possibly to do with the plane number, but more likely to do with the context they appear, i.e. what Types do they point at and are pointed at from?  Or maybe, T-A and T-9 are the same, but the sub-trees are the opposite way around - i.e. above=below, below=above?.

B, plane number, offset1, offset2
A simple 2 child node with a referenced Plane.  So why's it different to T-7?
Possibly that T-7 contains 'above the plane' and T-B has 'beneath the plane', or the other way around.

D, dx, dy, dz, rotx?, roty?, rotz?, scale, offset
7 floats, suggesting 3 for xyz, 3 for rotations & 1 for a scale (usually 1.0 - i.e. not scaled).  The three rotations are usually in the range -6.28 to 6.28, i.e. -1radian to +1radian.

This type seems to be a general transformation node - that is, all children of this node are changed by this node.

E, String offset, number
number has only been seen as 0
String offset gives the name of another 3DO file.  T-E's seem to follow from T-13's and I believe are used together to define the position of an external object.

>> Doh!  There's only four plane/offset pairs, then an offset, then -1 (Thanks Nigel!)

F, plane1,offset1, plane2,offset2, plane3,offset3, plane4,offset4, offset5, 0xFFFFFFFF (-1) 0,0,0,0, no. of following sets, sets...

where a set is: float, float, float, offset.
Where planes & offsets can be 0xffffffff and shouldn't be followed if they are.

I.e. 4 zeroes indicates there is no more data, 4 zeroes and a 3 indicate there is 3*16 bytes more.  The three entries listed as float could be x,y,z coordinates but I'm just guessing.
An example of a T-F would be F, (p,o), (p,o), (p,o), (p,o), o,-1, 0,0,0,0,4, (x,y,z,off), (x,y,z,off), (x,y,z,off), (x,y,z,off).

The offsets contained with the sets point to 3DO chooser nodes - T-13's.  I.e. a T-F might point to 4 other T-F's and 2 T-13's.

These nodes should not be treated as part of a single large tree.  They create loops if treated this way.  For more information see the Track part of this page.

This appears at the start of the PRIM section, the end, and in the collision part of a 3DO among other numerous places.
10, number of numbers, numbers...
T-10's appear to choose a set of numbers, but as they are used in different places, the meaning must need to be taken in context.  I.e. T-10's are used in collision trees, for .trk numbers, for sounds, etc.

11, number1, vertex, number3, no. of pairs, pairs(float, offset)...
Number1 would appear to be either 0 or 4.  If 0, the floats given in each pair are in decreasing size.  If 4, all the floats are zero.  Number3 is either 2 or 5 and usually 2 goes with 0 and 5 with 4, but there are some exceptions.

The vertex is the reference point used when calculating how far away the viewer is.

If the node was:  11, 0, 123, 2, 3 pairs, (100, tree1), (50, tree2), (0, tree3)
Then from infinity to 100 metres away then tree1 will be followed;  from 100 to 50 metres tree2 will be followed and 50 to 0 metres tree3.

13, dx,dy,dz, rotx?,roty?,rotz?, scale, offset
7 floats and an offset.  The offset points at a T-E, which chooses a 3DO file.

Found in lotus/lotus.3do & presumably other cars.

14, number, number.

So this Type looks like T-10 with unknown numbers

16, float, float, float, float, float, float, float, offset
Found in lotus/lotus.3do.  Almost identical to T-D & T-13, with 7 floats and an offset

17, no. of vertices, vertices...
Found in lotus/lotus.3do & other cars.  Seems to define an area via vertices.

19, v0, v1, v2, v3, v4, v5, v6, v7, offset
The v's are vertex numbers.  When drawn, these vertices create a 6 sided enclosed region:
[v0-v1-v2-v3], [v4-v5-v6-v7],
[v0-v1-v5-v4], [v2-v3-v7-v6],
[v0-v3-v7-v4], [v1-v2-v6-v5].

A03, vertex number, colour, string offset, float, int3
Looks like T-3 but for the cars.  Untested.

T-1f    Polygon - Textured

1f, 0, number (hex 05000000 or 0a000000), no. of vertices, texture x,y pairs..., vertex numbers...

Looks remarkably like T-81f, just with an extra number at the start.  Only found in car 3do's.

20, 0, 0, number (hex 01000000), no. of vertices, texture x,y, pairs..., vertex numbers..., vertex colours...

Looks remarkably like T-820, just with a few more numbers at the start.  Only found in car 3do's.

21, 0, number (hex 02000000), no. of vertices, texture x,y, pairs..., vertex numbers..., vertex normals...

Looks remarkably like T-821, just with an extra number at the start.  Only found in car 3do's.  The last set of numbers are normals for lighting effects.

815, colour, vertex1, vertex2
Probably the simplest primitive, 815's define lines.  These are used for things like telegraph wires and don't appear very often.

81B, colour, no. of vertices, vertex numbers...
Defines a polygon.  No texture mapping, flat shading.
There are no. of vertices vertex numbers at the end of this Type, e.g. 81B, FF204080, 4, 12,13,14,15.

81C, colour, no. of vertices, vertex numbers..., vertex colours...
Defines a polygon.  No texture mapping, possibly Gouraud shading (from the vertex colours).

81D, colour, no. of vertices, vertex numbers..., vertex normals...
Defines a polygon, No texture mapping, possibly fancy shading (using normals).  If no. of vertices is 6, there are 6 vertex numbers and 6 vertex normal numbers.  The numbers reference the XYZS & NORM sections.

81E, colour, no. of vertices, vertex numbers..., vertex colours..., vertex normals...
Defines a polygon, No texture mapping, Gouraud shading.  The normals are often all the same, but this is acceptable since they specify direction from a vertex.

81F, 0, colour, no. of vertices, x,y pairings for each vertex..., vertex numbers...
Defines a polygon.  Texture mapping using the xy pairings (floats),  flat shading if not textured.

820, 0, colour, no. of vertices, x,y pairings..., vertex numbers..., vertex colours...
Defines a polygon.  Texture mapping, Gouraud shading if not textured.

821, 0, colour, no. of vertices, x,y pairings..., vertex numbers..., vertex normals...
This looks like a polygon with texture mapping, flat shading, specular shading.

(did that make sense?  yes?  blimey!)