Forget the slots, they never mattered. There’s kinetic weapons and energy weapons. They are defined by their ammo types: bullets and hard projectiles vs energy (a gun that fires bullets vs a gun that shoots lasers, ammo clip vs battery pack). Heavy weapons would also fall under these types, but the game mechanics required a third weapon type so Bungie said “If the gun is really big, we’ll call it Heavy.” Arbalest is an energy weapon because it fires charged bolts of energy. The slot it’s in doesn’t matter. Kinetic energy is literally just energy generated from movement. Example: a bullet being fired and hitting you in the toe.