[Kobal2]

Good question.

I have always assumed, without much evidence for it, that it goes along the lines of:
- does this unit have a ranged weapon, yes/no ?
- if multiple units have ranged weapons, is this one closer, yes/no ?
- if multiple units have ranged weapons and are at the same distance, will attacking this one statistically cause more total HP damage, yes/no ?
- if all yes, pick at random.

The big if is on question n°3, specifically whether or not the AI considers shields and their chance to parry/add protection, size and the inherent chance to miss a smaller target etc... IIRC, the spell AI actually "casts" given spells at all likely targets, rolls and all, then picks the one location that affected the most HPs in total (good or bad - it'll bless giants over the same amount of human sized sacred for example) to cast it for realz. Presumably the archer AI is the same.

It will most certainly target Man's armoured crossbowmen (though possibly not prioritize them over, say, indy archers) and horse archers. Also slingers and javelineers (Ermor and Pythium's Principes are a neat way to mess with fliers with attack archer commands). Pretty sure it also goes for things like Green Lions, who are not exactly archers but do have a ranged weapon.