You can use it on any capital ship after one of your capitals have died. I used it yesterday to bonk a level 2 rapture to level 7 with 1 click.

Does it work if you scuttle?

Just put Res on autocast. It only fires if it can give a free level boost to one of your ships.

What it does is level up the target ship to match the level of a ship of that class which you lost in combat earlier in the game. You know the way you can buy the first few levels of XP - the benefit of Res is that it doesn't cost credits and gives the new ship more XP than you could buy for it. You still have to allocate ability points and pick between fighters and bombers, but it is cheaper.

It's hard to judge how useful Res is as it depends a lot on whether you're going to be losing ships and how easily and quickly you can get a level 1 replacement in to the action. Keeping the Res ship back at the shipyard is the best way to make sure the skill gets used and that you won't waste credits buying XP instead. For me it isn't really worth losing the Res ship from my attack fleets though.

Res lets you be very aggressive with high-level capships and put them at greater risk that you otherwise would. Without Res it would be worth some losses to an enemy to take out a level 7 carrier, but with Res they're only really scoring a level 1 carrier and that's not usually worth a big sacrifice.

Don't worry about the dead ship. The ability targets a live replacement of the same class. It really, really, is better set to autocast. All you need do then is watch for when you lose a capship and queue a replacement of the same kind of ship, then get it and the ship with Res to the same planet. Once there's enough antimatter the ability will fire and then you just respond to the level-up message by spending the ability points on your newly-levelled replacement ship.

There's nothing about Res that makes it more or less useful against either human opponents or AI. What does make a difference is whether you ever lose capital ships or even risk losing them. If you do lose ships sometimes or if you have to baby them along and sometimes lose tactical opportunities because of it then Res might be helpful. I think it is always worth one point myself, on the chance that I'll need it... but some games it doesn't actually get used, so if you can tell that in advance you'd be right to skip it.

Yes. It's not limited to boosting only one ship either (just one at a time). If you build two replacements for your lost level 7 battleship they both get level 7, although there is a lengthy cooldown on the ability.