The TargetMailboxDatabase parameter specifies the Exchange mail database to create new mailboxes in. One of the following ...

The TargetMailboxDatabase parameter specifies the Exchange mail database to create new mailboxes in. One of the following "friendly" names: database name; server name\database name; server name\storage group\database name.  If specified, the parameter implies that mailboxes should be created. By default, a mailbox is not created if one doesn't exist.