For example in the pic above, with the code 0021403
00??x0x: Options (21)
The ?? are the 3rd and 4th digits. His value is 21, which is Sedan (+16), Washer Fluid warning (+04), Brake Pad warning (+01). Those three give you 21 so that's what you put in. If you wanted to add the seat belt warning (+02), then you would ad 21+2 and enter 23 instead.
00xx?0x: Country (4)
Here the only change is in the 5th digit. In his case, it's 4, which represents Great Britain.
00xxx0?: Distance Impluse Number (3)
Only the 6th digits gets changed and in this case its 3 (don't mess with this one - affects the speedo)
Hope this kinda explains it...