3GPP 23.501 v20.2.0 — the document's own text

5.16.3.7 UE's usage setting

Taught in 21. Emergency calls, messages, voice and location (The 5G system architecture, in depth).

If the UE is configured to support IMS voice, the UE shall include the information element "UE's usage setting" in Registration Request messages. The UE's usage setting indicates whether the UE behaves in a "voice centric" or "data centric" way (as defined in clause 5.16.3.5).

A UE supporting IMS voice over 3GPP access connected to 5GC and that is EPS capable shall also support IMS voice over E-UTRA connected to EPC.

NOTE: Depending on operator's configuration, the UE's usage setting can be used by the network to choose the RFSP Index in use (see clause 5.3.4.3). As an example, this enables the enforcement of selective idle mode camping over E-UTRA for voice centric UEs.