How to enable a Custom Maintenance Page in B2C Commerce

Customer Initiated Maintenance

• When the site status is changed to "Maintenance" at Administration > Sites > Manage Sites, the Default-Offline controller is called for storefront requests for that site regardless of the request URL. SFRA renders the siteOffline.isml template when Default-Offline is called. Customize this template according to your needs.
• Notes
  • The Custom Maintenance Pages uploaded via Administration > Site Development > Custom Maintenance Pages have no relation to the maintenance page loaded when the site is in maintenance mode.
  • When a site is in maintenance status, all storefront requests are handled by Default-Offline, and the maintenance page is displayed in the storefront regardless of the client user. In other words, it is not possible, for example, to allow Business Manager users to browse the storefront in the usual way while displaying the maintenance page for end users (shoppers) simultaneously.
  • The same goes to the Online (Protected) status. It is not possible to display a custom maintenance page (Default-Offline) while simultaneously protecting the storefront with basic authentication when the site is in Online (Protected) status.

Salesforce Commerce Cloud Infrastructure Maintenance

• Refer to the official documentation: Maintenance Pages
• Note: There is a limit of 9 for the total number of zip files that can be uploaded for the custom maintenance pages. If you require more than this number, please combine all the folders within one zip file. Each of the folders must contain an index.html file. 
• See also How to create and upload a Maintenance Page for B2C Commerce sites