max class is the highest class weapon you can mount on that ship
optimum class is the highest class that allows you to fire the longest (IIRC, something like 15-20sec)

Chris Bates

It's not much important, I use to mount the highest weapon the ship can. It's as if the vendor says "That ship was designed to mount class-x weapons, but you can anyway mount class-x+2 weapons. The energy will not long so last, so you can run out of energy in middle of a doghfight, so it's your deccission what to mount"
Ey, wait 10 seconds to reload your energy banks it's no so hard

I've always found it slightly irritating that everybody else in the story line game -- Juno, King, all of the good guy NPCs -- fly ships with more powerful guns and shields than their ships are supposed to be allowed to use. If they can, then why can't I?

It's an interesting point: The universe it's made to be explored in the SP campaing, so, as you are going deep and deep in it, you can get better and better ships/equipment. If you take the Freelancer's universe from a realistic perspective, doesn't matter how good the liberty pilots are, them are just CRAP against the whole sirius sector (level 3 ships, what a joke for a Valkirie).
And we notice that the main characters are from the houses, mainly the liberty house. That means that them have to be flying in a house's fighter, as them WORK from that house. You are just a freelancer, and can do whatever you want with what you are piloting in.
So, how could we make that point balanced, as the liberty fighters are the space equivalent to paper-made airplanes to the rest of the ingame factions? Just trick'em. End of the story.