How can we know the meaning of parameter of psp API?

[cp25stack]

Searching into the forum, i can understand that we know the API's name and "NID"
from disassembly/reverse engineering the prx file in the firmware (and/or UMD).
But i still dont know how to know the meaning of parameter of the API.
As i understand when dissembly it, it will show only parameter type suchas...
0x4XXXXXX "sceXXXXXX" (int, int, bool...)
something like that?

From the tutorial, in the case of
sceGuDrawBuffer(GU_PSM_8888, (void*)FRAME_BUFFER_SIZE, FRAME_BUFFER_WIDTH);
how to know the define parameter "GU_PSM_8888"
And how to know is there another valid value for 1st argument of sceGuDrawBuffer?
Or does that defined values are discover from disassembler too?

[Bytrix]

The whole SDK was produced by decompiling system PRXs and analyzing the function calls in them. If you look through the ASM you can see what hardware registers are used for each inbuilt function and then the SDK adds an interface to those functions in C.