RMAN backups clarification - Incremental Level=0 (Cumulative/Differential)

What is the difference between the below RMAN commands? Note: They are Level 0. Also, these are all syntactically correct and acceptable to RMAN

1) "BACKUP INCREMENTAL LEVEL = 0 CUMULATIVE"
2) "BACKUP INCREMENTAL LEVEL = 0 INCREMENTAL"
3) "BACKUP INCREMENTAL LEVEL = 0"

I've read the TRMs..but they don't clarify the above question. Yes, I understand the difference between cumulative and incremental backups..but I don't understand the context in a "level 0" backup. I thought (and documentation states) a level 0 always included ALL the blocks that have ever been modified!

So what do commands "1" and "2" do? - compare the previous level 0 SCN to do a cumulative/incr backup respectively?!

AFAIK, no diff between all.

Thanks. No idea why RMAN syntactically accepts the Cumulative or Incremental parameter with a Level 0. I'd seen a bunch of folks post their scripts on Metalink with such parameters! It led me to wonder..hence the question.

Anyway, when I get the time I'll kick off all the below..will post if there's any significant difference in the backup size. Hopefully there won't be any!