I have an example, FYI:
Consider u had done a full database backup with RMAN, so it's a level 0 backup, I use this sentence:
BACKUP INCREMENTAL LEVEL 0 DATABASE;
And the level 1 backup sentence is:
BACKUP INCREMENTAL LEVEL 1 CUMULATIVE DATABASE;
or:
BACKUP INCREMENTAL LEVEL 1 DIFFERENTIAL TABLESPACE users;
By the way, u can reference Oracle doc(no:b14194).