3GPP 24.501 v20.0.0 — the document's own text
5.5.4.2 ProSe relay transaction identity (PRTI)
Taught in 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
Upon receiving a ProSe direct link establishment request from a 5G ProSe remote UE or a 5G ProSe end UE for establishing a secure PC5 unicast link as specified in 3GPP TS 24.554 [19E], the UE shall allocate an available PRTI value for the authentication and key agreement procedure for 5G ProSe UE-to-network relay or 5G ProSe UE-to-UE relay and associate this PRTI value with the 5G ProSe remote UE or the 5G ProSe end UE.
The UE shall release the PRTI value allocated to the authentication and key agreement procedure for 5G ProSe UE-to-network relay or 5G ProSe UE-to-UE relay when the authentication and key agreement procedure for 5G ProSe UE-to-network relay or 5G ProSe UE-to-UE relay completes or is aborted.