No one can. Or, that is to say, any ~one~ person can, but no two are likely to agree.
By the Rules as Written... um... you'll end up just as confused as you are now. Better just read this recent thread:
And there are many others that go more in depth than that one in the older pages of these boards. This question gets kicked around regularly, and the answers never get any clearer, and the best responders (not me) get tired of posting their same answers again and again.
The problem revolves around Parma (and MR and etc.) not being able to tell a magical "attack" from a passive magical "effect" that a mage comes into contact with, and the issues that creates. For almost any simple effect that seems perfectly benign, there is a very similar one that could spell doom. So, each Troupe and SG comes up with their own houseruled paradigm (or accepted handwaving) that addresses the problem to meet their tastes and needs.
(If anyone has a link to one of the more complete discussions, from back before the great burnout, please post, thanks!)