In the Earthdawn system, this was a fixed number based on your Perception.  Some Disciplines got very minor bonuses as they advanced.

In the Shadowdawn system, it usually means "resisted by Willpower and any Magic Resistance".   It's also a good clue that the relevant stat on the "attacker's" side should be Will or IQ.  SocialDefense, on the other hand, is usually an indicator for Charisma.