So called "mailbox disk" it's a old (now using as a spare) method for clustering interconnection and communication between two cluster "heads".
"Mailbox disk" it's a special area at a two of disk (by default one data and one parity, or one parity and one dparity disks), and usually resides at a first aggregate.
It's consume a very small amount of space (around KB or two)
Actuallly, it's just a flag area, one "head" can raise the flag - "I'm alive! Everything is OK!" for another one, partner "head", and vice-versa. It looks like e-mailing between heads, it's a reason why they calling it "mailbox".