This parameter specifies the local mailbox name to be used for the connection, and is equivalent to the -LMB parameter you would specify if you were connecting using the Datanet Terminal Manager.
This can be used as an additional user identification for host security. On GCOS8 systems it may also be used to generate a LID, while for IBM (OSF/Janus) connections it can be used to select a specific LU or LU pool.