The difference between disable mailbox and mailbox zap in Enterprise Vault is one leaves the metadata in the mailbox untouched and the other one removes it.
Basically during a disable mailbox operations, you configure the mailbox archiving status in the database to “do not archive anymore”. This leaves all metadata in place, such as custom policies and hidden messages.
A mailbox ZAP operations on the other hand will delete all hidden messages, folder archiving rules and so on. If the mailbox has not been disabled, the mailbox metadata will be redistributed by Enterprise Vault and archiving can continue.