Maintenance Mode -

Why to enable maintenance mode?

This is enabled to ensure optimal performance and reduce downtime during a patching session. It shuts down the workflow business event system and set up function security so that oracle applications functions are unavailable to users. This provides a clear separation between normal runtime operation and system downtime for patching.

Note:  Maintenance Mode is only needed for AutoPatch and other AD utilities not require

How Maintenance Mode enabled:
There are two way:

1. Using adadmin:
i)Source application environment.
ii)adadmin: Options 5, 1

When we enable maintenance mode using utility adadmin, internally it calls adsetmmd.sql script located at FND_TOP/sql location. 
When adsetmmd.sql executes, it sets a Profile Option 'Applications Maintenance Mode'(APPS_MAINTENANCE_MODE) to 'MAINT' to Enable 'Maintenance Mode' and to 'NORMAL' to Disable.

2.Using script directly(adsetmmd.sql):
i)Run script :sqlplus <APPS_Schema name>/<APPS Password>@adsetmmd.sql ENABLE | DISABLE

Query to know the status of maintenance mode:
sqlplus apps/apps
SQL> select fnd_profile.value('APPS_MAINTENANCE_MODE') from dual;

If returns 'MAINT', then Maintenance Mode is Enabled and unable to Login. 
If returns 'NORMAL' then Maintenance Mode is removed and able to login.