It's /almost/ like all of this stuff was made up, I mean written by a
lot of different people with different ideas about what the Imperium
actually is and does over several decades. Crazy, huh?
!!
My 'defence' of the standard rules (much though I rather like Jeff's additional detail as it's helpful to referees) is that these are only approximations anyway. The fun is in finding that for whatever reason your A class doesn't have quite the facilities you thought it might have
(maybe something's broken or damaged by the last lot of PCs to go through)
or you *can* indeed get x, y or z at a C class 'port (maybe it's *just* been installed and the rating hasn't been upgraded yet or for political reasons isn't going to be).
The UWPs cover a *lot* of ground (literally) in just a few letters/numbers and I think they do a great job of this. I'm not sure I'd want a High Guard style string of dozens of digits that's only readable with a decode sheet. (Though I'm aware that newbies can't usually look at a UPP or a UWP and 'see' the person/planet behind. But six digits + prefix/suffix of starport and tech level seems like a good trade off.)
So I think I'm saying, treat the starport type as you would government type or law level - or more explicitly tech level which we know 'varies', rather than size or hydro which is an easily measurable digit.
In general "A" means top notch everything there quite large and "E" doesn't. But there may be variations. YMMV. YSMV. (Your mileage may vary. Your starport may vary).
tc