A scm file is a binary file with certain structure. The common information about what this file is for can be found here. The scm file structure looks like the following:
(02 00 01)h + 32-bit int Jump to second segment byte Target game ('s' - San Andreas for SA) (Global vars) Space for variable saving (02 00 01)h + 32-bit int Jump to third segment byte Segment id 32-bit int Number of models (Model names) 24-byte model names * number of models (model 0 name is empty and therefore unused) (02 00 01)h + 32-bit int Jump to fourth segment byte Segment id 32-bit int MAIN size 32-bit int Largest mission size 16-bit int Number of total missions 16-bit int Number of exclusive missions 32-bit int Highest number of locals used in mission/exernal (Mission addresses) 32-bit addresses * number of missions (02 00 01)h + 32-bit int Jump to fifth segment byte Segment id 32-bit int Largest external size 32-bit int Number of externals (External data): 28 byte external data * number of externals 20 bytes Name of external 32-bit int Offset of external 32-bit int Size of external (02 00 01)h + 32-bit int Jump to sixth segment byte Segment id 32-bit int (Unused, always 0) (02 00 01)h + 32-bit int Jump to seventh segment byte Segment id 32-bit int Size of global var space 8-bit int Number of allocated externals (with 07D3, 0884, 0928 or 0929)? (unused, always 3Eh) 8-bit int Number of unused externals (HOME_BRAINS and AAA)? (unused, always 2) 16-bit int (Unused, always 0) (MAIN code) MAIN section, equal to size defined earlier (Mission code) The mission data, missions stored at offsets defined earlier
The AAA external script is very odd as the sixth segment is copied inside. The following bytes are just padding to align the file size to the nearest 2KB, like also in the working externals.