3GPP 24.501 v20.0.0 — the document's own text
6.2.1 Principles of PTI handling for 5GSM procedures
Taught in 6. Getting on the network, and getting a connection through it (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 UE or the network 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 request message or of the command message.
If a response message is sent as result of a received request message or a received command message, the sending entity shall include in the response message the PTI value received within the request message or within the command message (see examples in figure 6.2.1.1, figure 6.2.1.2, and figure 6.2.1.3).
If a command message is sent as result of a received request message, the sending entity shall include in the command message the PTI value received with the request message (see examples in figure 6.2.1.3).
If a command message is not sent as result of a received request message, the sending entity shall include in the command message the PTI value set to "no procedure transaction identity assigned" (see examples in figure 6.2.1.4).
