What really happens when you enable maintenance mode before applying patches

Maintenance mode provides a clear separation between normal runtime operation of Oracle Applications and system downtime for maintenance. Enabling the maintenance mode feature shuts down the Workflow Business Events System and sets up function security so that no Oracle Applications functions are available to users. Used only during AutoPatch sessions, maintenance mode ensures optimal performance and reduces downtime when applying a patch. For more information, refer to Preparing your System for Patching in Oracle Applications Maintenance Utilities.

You may also refer to Article ID 291901.1, which has more information about Maintenance Mode.