Maintenance Mode was introduced in AD.I patchset. Here's what the readme of AD.I (Article ID: 233044.1):
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.
Interestingly R12 has some additional steps to enable maintenance mode. Plese refer to Article ID 558274.1: Instructions for configuring maintenance mode for R12.
Blog dedicated to Oracle Applications (E-Business Suite) Technology; covers Apps Architecture, Administration and third party bolt-ons to Apps