Presence service

Presence service is a network service which accepts, stores and distributes presence information.

Presence service may be implemented as a single server or have an internal structure involving multiple servers and proxies. There may be complex patterns of redirection and proxying while retaining logical connectivity to a single presence service. Also presence service may be implemented as direct communication among presentity and watchers, i.e. server is not required.

The Presence Server, that reside in the presentity's home network, is able to receive and manage presence information. It is a Application Server and is located using SIP URLs.

...
Wikipedia