3GPP 24.501 v20.0.0 — the document's own text
B.2 Protocol errors (e.g. unknown message)
Taught in 5. Finding your way around TS 24.501 (CT1 — the group that writes what your phone says, in depth), 7. What a NAS message looks like on the wire (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).
Cause #95 – Semantically incorrect message
This 5GSM cause is used to report receipt of a message with semantically incorrect contents.
Cause #96 – Invalid mandatory information
This 5GSM cause indicates that the equipment sending this 5GSM cause has received a message with a non-semantical mandatory IE error.
Cause #97 – Message type non-existent or not implemented
This 5GSM cause indicates that the equipment sending this 5GSM cause has received a message with a message type it does not recognize either because this is a message not defined, or defined but not implemented by the equipment sending this 5GSM cause.
Cause #98 – Message type not compatible with protocol state
This 5GSM cause indicates that the equipment sending this 5GSM cause has received a message not compatible with the protocol state.
Cause #99 – Information element non-existent or not implemented
This 5GSM cause indicates that the equipment sending this 5GSM cause has received a message which includes information elements not recognized because the information element identifier is not defined or it is defined but not implemented by the equipment sending the 5GSM cause. However, the information element is not required to be present in the message in order for the equipment sending the 5GSM cause to process the message.
Cause #100 – Conditional IE error
This 5GSM cause indicates that the equipment sending this cause has received a message with conditional IE errors.
Cause #101 – Message not compatible with protocol state
This 5GSM cause indicates that a message has been received which is incompatible with the protocol state.
Cause #111 – Protocol error, unspecified
This 5GSM cause is used to report a protocol error event only when no other 5GSM cause in the protocol error class applies.