A SIP registrar is used to process SIP address or URI information from a user agent. This information is stored in a location service and can be later interrogated to determine how to route calls to the user. It can also be used for load balancing and providing system redundancy. The user-configurable files contain registrar information with an associated list of user names in one or more files, and can override default values for port, timeout of registration, user agent, and priority. These registrations are triggered by enabling one or more VoIP trunks.
An authentication digest challenge mechanism allows a registrar to identify the registering party and so stop third parties from being able to register as a user. This feature will work automatically if your registrar supports it and has it enabled, and can be used independently from secure SIP.
For further information, see Using a SIP registrar.