Here you can specify the sites into which the user can sign in using their user name and password credentials.
To assign the user to a site, simply click the Add sites button, check the appropriate boxes in the displayed dialog and click OK to save the changes.
The sites assigned here only limit access to the CMS Desk interface. Logging in on the live site is possible even for users who are not assigned to the given site.
This is intended to allow the separation of access privileges for content editors responsible for different websites.