It does not apply to non-FDO data, the first packet transmitted by the client (good luck reversing that one), init packets, and split packets, or the token "f2" (where the data is in raw form: 2x unsigned short -- the data for f2 invoke 40-1827 would be 0x00 0x28 0x07 0x23).

Data {
 2 bytes: Token ID
 2 bytes: SID (in most cases; see below for exceptions)
 2 bytes: microcode opcode
 1 byte: length of parameter to opcode
 x bytes: parameter
 2 bytes: new opcode..
 
 and so on.
}

Some opcodes are of type 2; in that case, the format is not
 2 bytes: AOL Opcode
 1 byte: length
 x bytes: parameter
but
 2 bytes: AOL Opcode
 1 byte: parameter

This only applies to what the client sends. The server's packets are munged in some way that has not yet been reversed.

Note on SIDs: in some client tokens (file transmissions (x* token IDs) mainly), the SID is absent. This does not follow any preset pattern, so it must be hard-coded for each token ID.