That is because Renegade Legion (and many older US- based productions) were drawing upon recent USN historical precedent of the time and our "oddities" in terminology over here across the pond that have since been brought into line with the terminology more commonly used worldwide.
Post WW2, the USN had
Destroyers (
DD) and also "
Destroyer Leaders" (
DL) that led
Destroyer Flotillas as squadron command ships. They also had
Destroyer Escorts (
DE) left over from WW2 that performed the same role as Destroyers, but for
Commercial Convoy Escort duty rather than
Fleet Escort (and thus did not need to meet fleet mobility standards, and were cheaper to build - the equivalent of the European "
Frigate" or "
Corvette/Sloop-of-War", depending upon size), and also "
Escort Destroyers" (
DDE) which nominally filled the role of Destroyer Escorts, but were larger and had Fleet mobility capability and thus could be pressed into service as Fleet Escorts at need as a "
Light Destroyer". They also had some big
Heavy Cruisers (
CA) left over from WW2 which were generally known as "
Gun Cruisers".
By the late 1960's/early 1970s, they had also introduced a new class called a "
Patrol Frigate" (
PF) similar to an Ocean Escort, but smaller.
So what does that have to do with anything? By the late 50s/early 60s, USN terminology generally referred to vessels by the following terminology:
- Battleship (BB) - (decommissioned)
- Gun Cruiser (CA) = ("Heavy Cruiser")
- Frigate (DL) = ("Destroyer Leader", or other nations' "Cruisers")
- Destroyer (DD) = ("Destroyer")
- Ocean/Destoyer Escort (DE) = ("Destroyer Escort", or other nations' "Frigates")
- Patrol Frigate (PF) = (other nations' "Frigates" or "Corvettes/Sloops-of-War" )
Note that the USN "
Frigate" was bigger than the
Destroyer and was comparable to other navies'
Cruisers. And the USN "
Gun Cruiser" was
much bigger than the rest of the world's
Cruisers. And USN "
Escorts" and "
Patrol Frigates" compared to other navies'
Frigates and
Corvettes.
By the late 1960s/Early 1970s there was what was perceived as a "Cruiser Gap" (purely perception and political, not reality in terms of actual capabilities), due to the naming convention, because the USN only had a few "
Gun Cruisers" left and still in service, and quite a few "
Frigates" along with destroyers and a number of "
Escorts". The Soviet Union by comparison had a number of "
Cruisers" in service. The feeling was that other nations would perceive the USN as under-equipped and under-gunned due to the difference in naming (we actually had a large number of Frigates, which were Cruiser-equivalents). So in 1975, Ocean Escorts (DE) and Patrol Frigates (PF) were discontinued and rolled into the new class of "Frigate" (FF), a small vessel, and the new and older Destroyers (DD) and Destroyer-Leader "Frigates" (DL) were reclassified as either Destroyers (DD) or "Cruisers" (C_ ) based on size and capability. Most of the new builds were Guided Missile equipped (CG, DDG, FFG, etc), and some were nuclear propulsion (CGN, DDGN, etc).
That is why
Renegade Legion classified their vessels the way they did. (And the reason why the
Star Trek "
Star Fleet Technical Manual" of the era classed the Federation Vessels as:
- Dreadnought (prototype) - (Mk X) (= Battleship)
- Heavy Cruiser - (Mk IX) (= Gun Cruiser)
- (Cruiser / Frigate) (?)
- Destroyer - (Mk VIII) (= Destroyer)
- Scout - (Mk VII) (= Destroyer/Ocean Escort / Patrol Frigate / Sloop-of-War)
- Transport/Tug - (Mk VI) (= Armed Escort)