Weapons can be equipped either as a Main Weapon or as Sub Weapons -- colloquially called as the Weapon Grid. The Weapon Grid provides the Main Character and party characters increased ATK and HP when equipped.
Weapons that can be equipped as Main Weapon are called Primary Weapons, and provide an added bonus to ATK and HP when equipped as either Main or Sub.
Any weapon can be equipped as a Sub Weapon, regardless of chosen class.
The weapon used when attacking.
The main weapon's appearance can change by selecting a weapon skin. Any previously aquired weapon (i.e. weapons with journal entries) of the same weapon type can be used as a weapon skin.
The skin selected will be visible on the main weapon's thumbnail on the Party screen. In addition, the appearance of the weapon held by the main character's sprite in quests, raid battles, co-op rooms, and on the Crew screen will all be changed.
The main weapon's skin can be changed by tapping the Skin button on the main weapon's thumbnail on the Party screen or by tapping the Equip a Skin button on the main weapon's details screen.
The Gladiator class line and Viking can equip an auxiliary weapon in the top-left slot of the sub weapon section. The weapon must be one of the class's speciality weapons of the same element as the main weapon in order for it (the Sub Weapon) to be used as an Auxilliary Weapon.
The weapon held by the main character's in-battle sprite and 2nd Charge Attack will reflect with equipped auxiliary weapon. Its skin can also be changed.