The Community Home page is shown to unauthenticated users in a public community and it allows you to specify a landing page for guest users without setting up redirects. Previously, the Community Home page was set in Visualforce or you had to create URL redirects to specify a Site.com page.
The Service Not Available page is shown when the community is not available due to a service outage. Previously, the Service Not Available page was set in Force.com. Use a static resource file to override the Service Not Available page.
In Community Management, go to to customize your community page assignments. Select a Community Builder (Site.com Studio) or Visualforce page to override the Community Home page.
Page selections made in Community Management override previous page selections specified in the Site.com and Force.com site properties. For more information, see Override Default Pages in Your Community with Custom Pages in the Salesforce Help.