These posts focus on the use of Diameter and SIP in an IMS / VoLTE context, however these practices can be equally applied to other networks.
The RFC’s definition is actually pretty succinct as to the function of the Server-Assignment Request/Answer:
The Registration-Termination-Request is sent by a Diameter Multimedia server to a Diameter Multimedia client in order to request the de-registration of a user.Reference: TS 29.229
The Registration-Termination-Request commands are sent by a S-CSCF to indicate to the Diameter server that it is no longer serving a specific subscriber, and therefore this subscriber is now unregistered.
There are a variety of reasons for this, such as PERMANENT_TERMINATION, NEW_SIP_SERVER_ASSIGNED and SIP_SERVER_CHANGE.
The Diameter Server (HSS) will typically send the Diameter Client (S-CSCF) a Registration-Termination-Answer in response to indicate it has updated it’s internal database and will no longer consider the user to be registered at that S-CSCF.
I’ve included a packet capture of these Diameter Commands from my lab network which you can find below.
Other Diameter Cx (IMS) Calls
User-Authorization-Request / User-Authorization-Answer
Server-Assignment-Request / Server-Assignment-Answer
Location-Info-Request / Location-Info-Answer
Multimedia-Auth-Request / Multimedia-Auth-Answer
Registration-Termination-Request / Registration-Termination-Answer
Push-Profile-Request / Push-Profile-Answer