Newbie Guide:
Upper and lower body armor is classified as either robes, light gear or heavy gear. Robes are for Mystics and that ilk, so we'll skip them and move directly to light and heavy gear.
When you select your occupation at Level 20, the armor you wear will influence your choice of skills. For example, the Palus Knight's Heavy Armor Mastery is a skill which increases defense only when heavy gear is worn. Likewise, the Assassin's Light Armor Mastery increases evasion only when wearing light gear.
Keep in mind, you are only considered to be equipped with a given type of armor if both the upper and lower body are equipped with the same armor type. Heavy Armor Mastery, for example, will not activate if you are equipped with, say, a Bronze Breastplate and Leather Pants.
These skills are only affected by your upper and lower body armor. Your helmet, gloves and other gear do not signify, so wear what you wish.
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