What does Exchange do during online maintenance?

Following is the list of activities done during an online maintenance for Exchange:

1) If the mailbox retention period is 1 month, and a deleted mailbox has reached 1 month, it is deleted during the online maintenance process.
2) If deleted item retention period is 1 month, and a deleted item has reached 1 month, it is deleted during the online maintenance process.
3) An online defragmentation is performed on the store which rearranges all the white space in the database. This does not compacts the database file size.
4) The online maintenance process queries the Active Directory to verify that the mailbox’s associated user account is still available.
5) For public folder stores, the online maintenance purges all messages marked for expiration.

Online maintenance and online backup cannot run simultaneously. If an online backup of a single store is triggered during online maintenance, the online maintenance will halt.