Enemy ships use builds. Say destroyer Y of faction Z uses 3 different builds, one build with 7 caps and 17 vents, one with 0 caps and 20 vents and another one with 12 caps and 0 vents. Now, another faction uses the same destroyer with different builds...
E: At least I think they do - factions have access to certain ships and weapons, IF the builds are strict and pick the exact weapons... Not sure if it's closer to the "build ships vs preferred weapons vs randomly picked build" - thing like for the player when you construct a ship with your heavy industry, and there are also your patrols... Uuugh. ^^
EE: As mentioned above, "free" remaining points are used for them (vents and caps) anyway I guess.
They basically use autofit the same way the player does with a limited pool of weapons known by the faction, including limited counts of said weapons. If the weapon desired by the variant (so called builds) isn't available it tries to put something else in. EX if the variant wants a heavy autocannon but there isn't one available, it will try another kinetic weapon like arbalest or hvd. I believe they have an unlimited number of low quality weapons though so there is always something to fill in the slot. It's worth mentioning a given hull can have variants not displayed in the autofitter, like the dem build for hammerheads or the builds the AI XIV fleet use. Factions also have variants they can prefer, again like the DEM hammerhead and persean league.
autofit is random with descriptors being assigned for weapons as to what weapons can be exchanged for what. There are numbers in each descriptor that describe the weapon's objective strength for the autofit. In case it wants to exchange a weapon to a stronger variant when it has the OP (for example from s-mods) or just in general
(at least that's how I think it work based of my experiments and autofit tag changes)
This might lead to kind of annoying situations in which weapons of some tactical utility that were intended in a build, get exchanged for something that has less flux efficiency or just different kind of utility. I can't necessarily call in specific examples. But a good one is Arbalest getting exchanged to Heavy Needler. Which at first glance might seem like an improvement. But for a Brawler that needs high hit strength weapons and is severely limited in flux capacity. And because of that can easily over flux itself. That's not a good idea.
There is also the infamous case of HIL getting exchanged to Paladin PD system, though that is quite rare. And you will only see it in like one in three Sunder or something
There is also the exchange of Flak Cannon to HMG
And I think Heavy Burst has a beam autofit tag, making it possible for it phase lances to be changed into Heavy Burst and vice versa. But Heavy Burst is rarely every equipped by NPCs by default. So you will usually see the former. Especially annoying for the Remnants if I recall correctly (though that might have been just me modding variants to include more Heavy Burst PD and then seeing it be exchanged into Phase Lances overfluxing Fulgents)
There are also strong autofit things for fighters. Which can also be quite annoying because different fighters perform different roles. A Gladius and Talons will not work the same way as Broadsword and Talons. But I don't think that tends to happen that often as carriers as of now in vanilla are generally rare. And those that aren't have a bit... Homogenous sets of fighters that are a bit more interchangeable in options without randomness screwing them over.
Though again, it often doesn't matter as much. Cause fighters in the end are extremely vulnerable. And it doesn't matter if it's a Claw or a Broadsword or a Gladius. They will get annihilated by flak just the same. Or Locust
