3GPP 24.501 v20.0.0 — the document's own text
6.2.4.4 Additional requirements of the UE acting as 5G ProSe layer-3 UE-to-network relay UE for IP address allocation
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
If a UE acting as 5G ProSe layer-3 UE-to-network relay needs to indicate "IPv6 Router" or "DHCPv4 Server & IPv6 Router" in the IP address configuration IE as specified in 3GPP 24.554 [19E], the UE shall support acting as a "Requesting Router" as described in IETF RFC 9915 [33D] to request additional IPv6 prefixes (i.e. prefixes in addition to the /64 default prefix which was allocated via stateless IPv6 address autoconfiguration as specified in clause 6.2.4.1) from the SMF as specified in clause 5.5.2 of 3GPP TS 23.304 [6E].
When the UE acting as 5G ProSe layer-3 UE-to-network relay UE requests additional prefixes using DHCPv6, the UE shall perform procedures described in clause 6.2.4.2a.