A、The backup set has exceeded the retention criteria set in RMAN and is eligible for removal. 

B、The backup set has one or more invalid blocks in it and is not usable for recovery. 

C、The backup set contains one or more tablespaces no longer in the database.  

D、The backup set contains one or more missing backup set pieces.

E、The backup set is from a previous version of RMAN and was not upgraded.