Access sites are how access controllers and doors are grouped together for easy management and monitoring. Each access controller needs to belong to a site. An access controller and its doors are tied together, so they move as one over to the new site.
Moving access controllers out of the default admin site shows how to create a new access site.
Note: Access levels, schedules, and exceptions are defined at a site level. Any doors moved to a different site will not carry their access levels, schedules, and exceptions with it. Make sure to create these in the new site and apply them to the doors to ensure they have their expected access.
Moving an Access Controller to a different Site
Select Devices and search for type:"Access Controller" to see a list of all your Access Controllers.
Select the controller you wish to move and its information will appear in the right pane. Select Site and you will see each of the sites you have access to move it to.
Choose the site to move the controller to, and the controller and its doors will move to the new site.
The moved doors will not have any levels configured, as these are site-specific. You'll need to apply them to the new site to ensure correct access. See Adding Access Levels and Schedules/Exceptions for information on applying or creating these.