3GPP 24.501 v20.0.0 — the document's own text
D.8.4 Unknown or unforeseen message type
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).
If the UE or the network receives a UPDS message with message type not defined for the UPDS or not implemented by the receiver, it shall ignore the UPDS message.
NOTE: A message type not defined for the UPDS in the given direction is regarded by the receiver as a message type not defined for the UPDS, see 3GPP TS 24.007 [11].
If the UE receives a message not compatible with the UPDS state, the UE shall ignore the UPDS message.
If the network receives a message not compatible with the UPDS state, the network actions are implementation dependent.