Polycom MGC Administrator Administrator's Manual page 584

Appendix E: IP Network Components
E-10
To: header field and forwards it, the redirect server only answers with the
information—it does not forward the request to the correct address. The
initiating party then uses the information provided by the redirect server to
reissue its Invite directly to the correct address.
Location Server
H.323
N/A
A location server is a server installed with an abstract service known as a
location service, which provides the address bindings for a particular domain
of User Agents to the devices they are logged on to. The information stored
and retrieved from the location service to/from the proxy or registrar is
transferred via a non-SIP protocol.
SIP
Optional
(recommended)