In case of incremental backups RMAN takes the backup of only modified blocks.
The different levels of incremental that are possible are :
1.level 0(complete backup)
2.level 1(incremental)
3.level 2(cumulative)
Level 0 is the base backup for next incrementals.
If we issuse level 1 without level 0, by default RMAN takes level 0 backup.
From 10.2.0.4, levels greater than 0 and 1 have become obsolete.
RMAN> backup incremental level 0 database;
RMAN> backup incremental level 1 database;
RMAN> backup incremental level 2 database; ---> earlier
RMAN> backup incremental level 1 cumulative database; ---> 10.2.0.4
RMAN> backup as compressed backup set level 0 database;
