There are two types of maintenance modes for a store: soft and hard.
Soft maintenance mode shows a maintenance message, written by the admin, on the store. This is good to use if the store will be unavailable for a period of time, such as going on vacation.
Hard maintenance mode shows a message for the store and the storeadmin.
Soft maintenance mode
Starting in 10.4.2
In storeadmin > Site Settings > Maintenance, enter in the maintenance mode message. Since a full featured editor is being used, pictures can also be entered.
Click 'Update Variables' to save and enable maintenance mode.
To disable maintenance mode, delete everything in the Maintenance:
Hard maintenance mode
Starting in 10.3.0
In the config.php file, add this line to enable maintenance mode:
A message will appear for the store and storeadmin:
This site is undergoing maintenance. It should be back up soon
will appear. To disable, change true to false: