Whether missile or melee fire, it is generally not possible to select a target in the swirling melee. For example, in a battle against 15 goblins, the PCs will typically be unable to pick which goblin they attack.¹⁾²⁾

However, a target (or targets) can be selected by intelligent creatures in the case where the target is “unique”. For example, if the target is classed, or a leader-type, or wearing some definitive mark. In such a case, the two parties become locked in a duel of sorts and will only be able to target one another until one side has been defeated. This isn't quite right, we need to see what this DMG 70 section is trying to say. ³⁾

All individual missiles should have their targets individually determined as per DMG 63. Every shot from a weapon with 2+ RoF⁴⁾ does not automatically target the same combatant.

When attacking a mounted target in melee, the attacker may choose whether to target the mount or the rider.

Sweep attacks are possible against mounted enemies riding mounts of 1 HD or greater only if the attacker is also mounted, or has some other way to completely negate the combative effect of the enemy mount (e.g. perfect magical hovering). Otherwise, the presence of the 1 HD+ mount makes sweep attacks against the rider impossible.

In the case of unintelligent creatures where there is no clear choice, a random target can be determined with a d6:

When attacking a mounted target with missile fire, a random target should always be determined in the above way.

This d6 roll works in cases of a M sized rider and a L sized mount, such as a PC and a horse. The die roll should be adjusted appropriately in cases of a larger size category difference, taking into note the rules in the DMG for the random determination of missile targets on DMG 63.

In cases where height is a significant factor, that should be taken into account. For example, a kobold with a short sword attacking a man riding a large, old, red dragon should always target the dragon.

See Spellcasting.