3GPP 23.501 v20.2.0 — the document's own text
6.3.6.4.1 General
Taught in 6. How one function finds another (The 5G system architecture, in depth).
UE initiates PLMN and non-3GPP access node selection for emergency services when it detects a user request for emergency session and determines that untrusted non-3GPP access shall be used for the emergency access.
When the UE supports connectivity with N3IWF but does not support connectivity with ePDG, as specified in TS 23.402 [43], the UE shall perform the procedure in clause 6.3.6.4.2 for selecting an N3IWF.
When the UE supports connectivity with N3IWF, as well as with ePDG, as specified in TS 23.402 [43], the UE shall perform the procedure in clause 6.3.6.4.3 for selecting either an N3IWF or an ePDG, i.e. for selecting a non-3GPP access node.