These are all useful comments -- I appreciate the thoughts. T5 is the only official source that I am aware of for the armed packet, and I'll use v5.09 as reference. I think some of the incoherence around the ship mission is baked into T5. While the name of the ship type ("packet") implies a mail or courier service based on historical usage, the actual T5 descriptions suggest passenger transport.
Packet appears as a specific mission associated with passengers on page 286. The full taxonomy is Commerce/Merchant/Unscheduled/Passenger, and packets are assigned code U. A Type A Trader, by contrast, is Commerce/Merchant/Unscheduled/Cargo.
This is reiterated on page 287: "For unscheduled routes: a Packet carries primarily passengers, a Trader carries primarily cargo, a Transport carries primarily freight."
We have an illustration on page 738, and the following description: "
Armed Packet (UF-CA33) in [sic] unscheduled passenger service for those in a hurry. The crew can be trusted, but these little ships still find themselves jumping in harm’s way."
And that, as far as I know, consists of all canonical information there is on armed packets.
Keep in mind, this design is a translation of
Rob Eagleston's T5 design. Given that Rob is close to Marc, I thought that was a pretty good place to start. Rob included both mail and passenger missions in the design -- which seems reasonable to me. So there are staterooms, a 1 ton mail vault, and barracks for the the ship's gunners (which are required for mail contracts). I suppose they could just be called "Gunners' Quarters" instead of barracks, if that is a sticking point.