CPEOLL is a subset in which you permute your last layer corners and orient your last layer edges simultaneoulsy. You put the cube into a special state making it able to be solved only using R and U moves (or L and U if you prefer those moves instead), or a 2-Gen state. This can be useful to force a certain set of ZBLLs that are only 2-Gen. If practiced enough it can be extremely useful for OH (one handed) solves.