All settings for Store Maintenance module you can find at
By changing value of option “Enable store maintenance mode” you can turn on and turn off maintenance for your store.
With option “CMS page to display while maintenance” you can specify page to display on frontend. You can select any of existing CMS page. Magento creates page “503 Service Unavailable” during module installation. It is selected by default. You can modify this page any way you want or select other from list.
Access to frontend
Obviously, while you updating website you need access to frontend and check how changes affect it. Also you want forbid access to frontend for the other visitors. You can do this by restricting IPs.
Option “Use Magento allowed IPs” with value Yes tells Magento allow
access to frontend for list of IPs that user specify in default Magento setting
You can set more IPs with access to frontend in field “Additional allowed IPs”.
Maintenance page header
According to Technical note you may want to inform search engine when maintenance mode ends and search robot can crawl actual content. Set option “Inform search robots when store back online” as Yes to do so. Then you have to set “Maintenance end date” and “Maintenance end time”.
If option “Inform search robots when store back online” is Yes, then
Retry-After appears in response header.