Any Messaging system can contain multiple sites. Each user is associated with a site and each site has own properties including:
Access number
Extension length
Mailbox length
Auto Attendant
Where sites are used, the directory for the site defaults to only users within the site. However, directory entries from other sites can also be included
Each application role can only belong to one site and has its own configuration such as language and time zone, which should be consistent for all servers in the site.