The "Combat - Attack x - Weapon Type" field determines which of the other Combat fields do and which don't have an effect on the attack, affects how abilities that differentiate between melee and range interact with the unit's attack (Vampiric Aura, Thorns Aura, Trueshot Aura, Spiked Barricades, Spiked Carrapace, Frost Armor, etc.) and whether certain abilities can be used in combination with the attack (Cleaving Attack, Orbs, Searing Arrows, Barrage, etc.).
Missile - Basic ranged attack; the unit launches a projectile that deals damage to a single target.
Artillery - The unit launches a projectile at the position of the target (not at the target itself) which can damage multiple targets in an area around the primary target. The attack can miss if the target leaves the attacked area before the projectile lands.
Units with this attack type also get the "Attack Ground" command, which allows them to effectively target multiple targets in the chosen area.
Units in which the "Combat - Death Type" field is set to "Can raise..." don't play their normal death animation and don't leave behind a corpse when they die from an artillery attack; instead, the effect defined in the "Art - Special" field of the dying unit is played at its position.