3GPP 24.501 v20.0.0 — the document's own text
D.1.2 Principles of PTI handling for UE policy delivery service procedures
Taught in 5. Finding your way around TS 24.501 (CT1 — the group that writes what your phone says, in depth), 4. Inside TS 24.501, the document at the centre (CT1 — the group that writes what your phone says, overview).
When the PCF or the UE initiates a procedure, it shall include a valid PTI value in the message header of the command message or the request message. When the UE initiates a procedure, the UE shall use a PTI value in range between 01H and 77H. When the PCF initiates a procedure, the PCF shall use a PTI value in range between 80H and FEH.
When the PCF initiates a transaction related procedure (i.e. a procedure consisting of more than one message and the messages are related), it shall include a valid PTI value in the message header of the command message.
If a response message is sent as result of a received command or request message, the UE or the PCF shall include in the response message the PTI value received within the received command or request message (see examples in figure D.1.2.1, figure D.1.2.2 and figure D.1.2.3).
If a command message is sent as result of a received request message, the PCF shall include in the command message the PTI value received with the request message (see examples in figure D.1.2.3).
NOTE: In earlier versions of this protocol, the UE can include in the response message a PTI value which is not the same as the one received within the command message, and therefore the PCF could not associate the response message from the UE to the command message sent.