We have a Defiant LC, Dictator C, Mars BC and Exorcist GC with that special rule.

It is said they have a built-in jovian bay - but which one, escort or landing? If landing, how can it be fitted on LC?

Please give me a link to errata or FAQ, or confirm that it was never specified in the rules.

It's in living errata 1.4 p. 10 (being most up to date, I believe): Exorcist and Mars have landing while Defiant have escort. I don't have anything specific about Dictator but I think they should have landing ones.

Never specified in the books, but I'd probably handwave landing bays for all of them. Or, since it was apparently errata'd, I might consider upping the strength of each built-in and non-removable component by one or increasing the number of squadrons that can be stored and/or held ready at a time and/or launched at a time. But only for those instances of the component that are built into the ship and non-removable, ie, if you went and installed a second landing bay on a broadside, it won't get the bonuses.

As for how/why ... these are built into the ship and non-removable - they should be better than versions that can be removed.

That is, a purpose-built, dedicated carrier hull is going to be better at being a carrier than a ship of the same approximate size that was converted to being a carrier or a ship that was designed to be something other than a carrier that has carrier capacity tacked on after/in addition to it's original function. Think about the early days of aircraft carrier development - the cruiser/carriers weren't that great, and it's not until purpose built carrier only carriers that they come into their own.

You'd be sorry. Rationalize it all you want. You have an extremely powerful squadron there already. The GM isn't going to be able to reasonably challenge that party in space with anything less than a deus ex xenos machina race or a battle group of 5+ capital ships.

That's ok. Just understand that you're now going to be playing space battles that involve 12-20 ships, and those take quite a bit of time to adjudicate.

Carriers are extremely powerful under the current rules. I'm creating house rules to stifle them, not give them even more advantages.

And there's no way in my next campaign that I'm even going to present the party a situation that is going to allow them to sail around with a squadron including a handful of capital ships. I'd rather throw pirate squadrons of raiders at them. They blow up more easily and are therefore harder to salvage, limiting party growth. Far too many groups out there running around with grand cruisers escorted by battle cruisers, making the Winterscale and Chorda dynasties look incompetent for having their skimpy fleets of the same size after centuries of growth.

I hate to harp on FFG's editing, but it can be so easy, sometimes. It says "Jovian-pattern something that starts with L" leads me to think it meant "Landing", and they said "Launch", then copy-pasted it a couple more times. Otherwise, they maybe mean you pick, based on your ship's needs. It was an easy way to say "your ship has to launch bays you can't get rid of, but we will give you a modicum of flexibility, based on your hull, desires, and available resources." Mechanically, the two aren't so different, and no so much that it hurts to say "you choose."

Mechanicaly landing bay can store and launch twice as many attack craft. Not so different my stern.

My apologies, but what i more meant is that, with three different size classes of ships between the examples, though the GC and BC can use Cruiser parts, of course, and having the light cruiser only having one size option, it was their way of saying "here are two spots. You can ONLY put Bays in them, and we like to copy/paste, so it's going to read the same everywhere, for uniformity." You still have to PAY at ship building for whichever kind you take, so it's not like they are giving you two bigger bays free. If you are shelling out for a cruiser, or bigger, though, they are giving you some flexibility. What can you afford to fit? They each need power, and each has a different space size, since, as you said, they obviously hold a different number of things (sorry about that, again), but you can pick. It says "Jovian" to hedge out the Lathe bay and Holding bay, which I think is only for transports, but only the Light Cruiser is limited to one, and copy/paste, yeah, I already said that.