In the ship designer, you can create ships with various weapons and defences. This is not exactly news However as you know, when you fight fleet battles the enemy targets those ships which have the highest threat level, using a formula of attack/(defence + HP) to calculate that threat level.

Now if you're going to have all the same ship in a fleet then I'm pretty sure that they'll pick on the greenest one since they'll have the least HP. But if you have more than one kind of ship in a fleet, it can be useful to figure out in advance what their threat levels are so that your expensive cap ship with all the fancy weapons doesn't get wailed on first. That is, you want your cheap throwaway fighters to have a better threat rating so they can effectively escort the cap ships.

For example, let's say you have a medium-hulled cap ship with an attack of 12, 2 points of defence, and the standard 16 hit points. Using the formula above, it has a threat rating of 0.667 and so if you were escorting it with a tiny-hulled fighter, the cheapest escort fighter to make might be one with 5 points in attack and none in defence (Threat rating: 0.833) - if it only had an attack rating of 4 then it would get the same rating as the cap ship.

Now if that escort fighter survives a few battles and gains hit points (let's say two, bringing it up to 8 HP) then this could present a problem. Assuming for the moment that your cap ship didn't gain HP at the same rate, or you're changing assignments and your veteran escort is being paired with a rookie cap ship, this might mean that your enemy will ignore the escort and go for the cap ship instead. In this example, either the cap ship has to have two more added to defence or hitpoints, or the fighter has to be upgraded by at least one attack point, to 6 or more.

The GalCiv 2 wiki (where I read about how ships in fleets are targeted) says that you can test this out by placing the escort and cap ship in a fleet together - if the escort appears in the list above the cap ship then all is well. That said IMHO it's a good idea to figure this out in the ship designer before you start rolling them off the production line