6.3.2. Per-Packet User Signal
In addition to symbol_user, per-packet user signals (packet_user) can also be declared on the interface. Packet_user can be of arbitrary width. Unlike symbol_user, packet_user must remain constant throughout the packet, i.e. its value should be set at the start of the packet and must remain the same until the end of the packet. This restriction makes the implementation of the data format adapter simpler as it eliminates the option to replicate or chop (wide source, narrow sink) or concatenate (narrow source, wide sink) packet_user.
If a source has packet_user and sink does not, the packet_user from source remains dangling. In such a case, the system designer must be careful and not transmit any critical control information on this signal as it is completely or partially ignored.
If a source does not have packet_user and the sink does, the packet_user to sink is tied to 0.