The distinction that I would make here is a question of Law ... as in which side of the Law you're operating on.
Under my proposal, P coded ships would be operating under color of law, while R coded ships would effectively be outlaws.
So the P ships are the system defense forces and the R ships are the pirates (in the wet navy sense) at the paramilitary organization level.
But, and excuse me to ask, who would put a mission code that reveals you as illegal?
I mean, we all know there may be pirates, and that's a ship's mission, but for its very nature, it's one to keep hidden, not to reveal in your code definition. That would be, IMHO, as stting you're a thief or an assassin in your linkedin page...
Frigate usually denotes a ship class, escort is a mission type. And therin lies the problem with the mission code shorthand - the conflation of ship class and ship mission.
Sure, but the frigate class ships mission has changed with time. At age of sail, they filled the role latter taken by cruisers (scouting, raiding and convoy escort), latter becoming more a light patrol ship, and currently a nearly captial ship in many a Navy..
What's now the distincltion (name aside) from a Frigate to a destroyer, or even a cruiser?
Some DDGs are large enough to be named cruisers, being kept as Destroyers just for political reasons, and the roles of the three classes usually blurr...
And I guess space navies will need different roles (ev en myabe some ones we cannot foresee now), and, while the classes may keep due to tradition, the class blurring may even increase.
I guess those classes are mostly for Solomani influences Navies, as the Vilani might have other definitions (be it by roles, by their own traditions, or whatever it could be), Zhodani still others and non-human races still others, maybe derived from their own needs...