Specification

R5AC is the name of an in-house cheat detection software. It seems to be made by a team at respawn entertainment, although there is no public information about it anywhere on the internet. 

Where is it?

It's located in the main game executable, r5apex_dx12.exe. Sometimes you will encounter entire functions that will be related to it, other times it'll be some inlined code in some important game/engine functions.