Monk class feature.
Prerequisite: Base Reflex save +3.
Prerequisite: Base Reflex save +7.
Evasion should be locked behind a character’s base Reflex saving throw. Sources indicate you can gain evasion initially as early as +3. +7 reserves improved evasion for characters with exceptionally good reflexes.
Evasion and improved evasion typically stack between classes. There is no benefit to gaining it a third time. No class gains it before 2nd level. Generic class bonus feats include the same prerequisites.