User connectivity is affected as follows when in maintenance mode:
With Server OS machines, users can connect to existing sessions but cannot start new sessions.
With Desktop OS and Remote PC Access machines, users cannot connect or reconnect once the machine is in maintenance mode. If they are already connected, then they stay connected until they next disconnect or log off.
Machines are available for user connections when you take them out of maintenance mode.
ref: http://docs.citrix.com/en-us/xenapp-and-xendesktop/7-5/cds-manage-delivery-groups-wrapper/cds-put-desktop-into-maintenance-mode-rho.html