NPPL is uncapped semi(ie the gun can fire as fast as you can pull the trigger)I think.
Capped semi is semi automatic capped at a specific number(usually 15 for psp)
PSP is 13BPS(or 13.33 for those who want to nit pick)ramping. I think it's either three or five shots at a specific rof then it ramps.
NXL is three shots(I think it's three), then full auto till you release the trigger.
Millenium is either 15 or 17(17 I think) ramping. It's harder to engage than psp.
Basically, read the description in a tadao manual or tournament rule series.