>>10395184"witch" refers to pretty much all psykers, part of the ordo hereticus' job is to test psykers and figure out if they're useful for anything, if they aren't useful they're sent to terra to feed the astronomicon.
"mutants" are humans with mutations that are deemed too deviant to be tolerable, this is somewhat arbitrary and all that really differentiates a mutant from an abhuman is that the inquisition deemed them impure and worthy of extermination, probably because they not useful or their mutations are believed to be caused by chaos taint rather than adaptation to their planet.
"heretic" is pretty much any human that opposes the imperium or the imperial cult: rebels, apostates, and yes chaos cultists too.
