New to Lync Server 2013 is the concept of server-to-server authentication, which allows for Lync to interact with Exchange Server 2013 and SharePoint Server 2013 on behalf of users. This authentication is done through a standard called Open Authorization (OAuth) where servers are granted a level of trust between each other via security tokens. There is no requirement to turn on this authentication feature, but it does enable a number of new features.
Server-to-server authentication can be used only between Lync Server 2013, Exchange Server 2013, and SharePoint Server 2013. This means that users must already have Exchange Server 2013 mailboxes in order to take advantage of the following features.