Units are Equipment used by the Player primarily to increase defensive stats such as MEL Def, RNG Def, TEC Def, etc., although certain Units do have inherent offensive stats in them.
There are four types of Units currently in-game: Back Unit, Arm Unit, Leg Unit, and Sub Unit. In addition to this, pairing up or having a set of a Unit and/or Weapon Series can give additional stats called Set Bonus.
Player Damage Resistances
There are about four categories of resistance in-game: Melee, Ranged, Technique, and Elemental Resistance, which compromises of the six Elements; Fire, Ice, Lightning, Wind, Light, and Dark Resistances. Although, it is common to assume that attacks can be a hybrid, that of which can be Melee and Elemental, the game only takes one type into consideration for the computation in regards to resistances.
Unlike the Back, Arm, and Leg Unit categories, Sub Units are able to be equipped in any of the three Unit slots, allowing you to easily fill a Unit slot if you are lacking a specific type of Unit. However, due to their flexibility, Sub Units do not physically display on the player, and they are typically weaker than Units of a dedicated type.