From ESOUI Wiki
This function sets one of the player's outfits as active.
- A number between 1 and GetNumUnlockedOutfits() indicating the outfit slot to equip. If this number is out of bounds, the game will display an error notification in the corner of the screen. The value nil will cause the call to do nothing, while other values of the wrong type will throw a Lua error.
-- Every character has at least one outfit. -- EquipOutfit(1)