Units are Equipment used by the Player primarily to increase defensive stats such as MEL Def MEL Def, RNG Def RNG Def, TEC 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 Back Unit, Arm Unit Arm Unit, Leg Unit Leg Unit, and Sub Unit 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: MEL Resist Melee, RNG Resist Ranged, TEC Resist Technique, and Elemental Resistance, which compromises of the six Elements; Fire Resist Fire, Ice Resist Ice, Lightning Resist Lightning, Wind Resist Wind, Light Resist Light, and Dark Resist 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.

Sub Units

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.