A.15.1- Evasion is not a specific ability, but is instead a category of abilities which limit what creatures can be assigned to block a creature. [D'Angelo 1998/02/03]
A.15.2- Evasion abilities are cumulative. In order to be assigned as a blocker, a creature must not be subject to any of the evasion exclusions. For example, if a creature had "cannot be blocked by black creatures" and "cannot be blocked by walls", then it could only be blocked by a creature that was both not black and not a wall. [CompRules 2001/07/23 - 501.2]
A.15.3- A creature may also have an ability which restricts its own blocking assignments. The rules for evasion apply to such abilities. [CompRules 2001/07/23 - 501.3]
A.15.4- These are static abilities (see Rule T.3) that modify the declare blockers step of combat (see Rule C.4). [CompRules 2001/07/23 - 501.1]
A.15.Ruling.1- The abilities can be written in either a positive or negative sense. For example, "this creature can only be blocked by walls" or "this creature cannot be blocked by non-walls". When combining evasion abilities, it's often easiest to put them in the same sense. [D'Angelo 1999/05/01]
A.15.Ruling.2- Evasion abilities only apply to blocker assignment (see Rule C.4). If a creature becomes a blocker by a means other than being directly assigned, then it is not subject to any evasion abilities. For example, if a creature blocks one member of a band of attackers, it becomes a blocker of other members of the band regardless of any evasion abilities those other members have. See Rule A.8.8. [D'Angelo 1998/02/03]
A.15.Ruling.3- An attacking creature with an evasion ability may not "turn off" the ability and choose to be blockable. [D'Angelo 1998/02/03]
Note- Fear (see Rule A.17), Flying (see Rule A.21), Landwalk (see Rule A.27), Protection (see Rule A.31), and Shadow (see Rule A.34) are evasion abilities.