C2MS13 — Why is Data optional in CCSDS Transfer Frame?

In Telemetry CCSDS Packet, DATA is required.

Transfer Frame Message has an optional DATA field, doesn't make sense for it to be optional, since the message needs to carry the transfer frame within it. Similarly, DATA is optional in TLM CCSDS CADU Frame and TLM TDM Frame. In all these listed messages, the message would not carry anything but info about the frame unless DATA were present, so should be required, in my mind.

Not concerned about backward compatibility, because it is already de facto required.

Note that TLM CCSDS Frame also has DATA as optional, but this message is deprecated, so leave it alone.