if a frigate and sub come into range of an opponent sub.
the opponent will fire on the first of those 2 that come into range.
and won't change target until the unit is dead
so : if frigate come into range first, it will tank the damage of the opponent sub, while your sub will take down the opponent sub.
of course you can ask your sub to re-target to kill the sub and not the frigate. But i feel like it would be more usefull to make the subs re-target by itself when a sub come into range.