Perception Challenge (PC)

Your Perception Challenge (PC), or the score your opponent needs to overcome to remain hidden from you, or to not be noticed

Perception Challenge (PC) = 10 + M(Perception)

This is very similar to your passive perception, which is

psv(Per)

psv(Per) = 8 + M(Perception)

They are similar in that neither rely on die rolls, because they rate your liklihood of noticing something in reaction to an event. The Preception Challenge is slightly higher (based on 10) because it rates your ability to notice something, when someone else is doing something unusual or challenging in your vicinity. Passive Perception reacts to events that are less focussed on you, even quite neutral events.