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

5.16.3.12 HSS discovery and selection

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

HSS discovery and selection functionality is used by the I-CSCF/S-CSCF/IMS-AS to select an HSS that manages the user's IMS subscriptions and has the ability to serve the IMS services for the UE, see clause AA.3.3 of TS 23.228 [15] and clause 6.3.1 for details.