Hey Questions Hope Someone can answer!

[95Blazer]

Hey, I'm actually asking for a buddy he just got the program and hardware to mod chips and well I own a 95 Blazer 4.3l Vortec, Auto tranny, OBD II system. Was wondering how you know what to change for timing, fuel, rev limit, ect...? well to be more precise i need to know what hex does what, so i need some info on this. I've been reading but cant really find anyone asking this question. thanks!