Armor Skills are attacks, buffs, or utility options included with specific armors, usually activated by clicking the skill button below your spells while in combat.
This guide will attempt to cover the main differences between the offensive skills found in the game.
As a precursor, Elemental Compensation, aka. Ele Comp is when an armor has a skill that is locked into one element and receives a boost depending on the armor's elemental resistances. Armor skills locked to deal damage of the same element they defend against result in higher Ele Comp values.
Old Lean skills are treated like player attacks, replacing your weapon that you equip with the "Weapon" summoned by the skill. This leads Old-Lean skills to ignore the bonuses on your weapons but they are boosted by a majority of buffing items.
Skills of this type are generally preferred due to the multitude of ways that their damage can be boosted and that their Ele Comp is typically used to increase their damage by a substantial amount.
Weapon based skills use your weapon as a basis for the attack. They can use the weapon's element, type, or both to determine their own attack stats.
Like Old-Lean weapon based skills are modified by any boost that would increase the damage of a normal attack.
Weapon based skills also have a tendency to use the Ele Comp they receive to reduce their cost, rather than their damage. leading them to often have lower damage numbers than Old Lean skills despite receiving similar boosts from gear.
Many armors with weapon based skills apply no sp cost or damage bonus when a weapon's special attack occurs, leading players to greatly prefer 0 proc weapons due to avoiding this decrease in damage while also gaining a x1.08 bonus for having no special at all.
While Spell Based armor skills appear very similar to Old Lean at first glance, they are treated like a player is casting a spell rather than attacking with a weapon. Similarly to Old Lean, their Ele Comp is applied to damage.
Because of this difference Spell Lean skills are not always modified by the same gear, and often receive half of the bonus an item would normally give. They tend to be weaker than old lean and weapon based skills due to the limitations in terms of boosting their damage.