Difference between full and level 0 rman backups?
The RMAN backup concepts guide (link for 11gR2, but the concept hasn't changed):


  The only difference between a level 0 incremental backup and a full backup is that a full backup is never included in an incremental strategy.


Full and level zero backups copy all blocks that have data (and more if you're doing image copies), so they are both "full" in that sense.
 But if you want to do cumulative or differential backups, the starting point for those must be a level zero incremental backup.