userlib question

I'm probably getting in way over my head but I am interested in trying my hand at creating a userlib. I downloaded the example from the specs page and looking at the decls file what do the @12 and @4 refer to for each function?

DemoLib_InvertBits( bank*,offset,byte_count):"_InvertBits@12"
DemoLib_ShuffleString$( str$ ):"_ShuffleString@4"

@12 means that it has a bank pointer called bank (4 bytes), an integer called "offset" (4 bytes) and another called byte_count (4 bytes). 4+4+4=12.

@4 means that it has one parameter called "str" which is a string pointer that is 4 bytes long.

Makes sense now, thanks Beaker.