Units are Equipment used by the Player primarily to increase defensive stats such as S-DEF, R-DEF, T-DEF, etc., although certain Units do have inherent offensive stats in them. There are four types of Units currently in-game: Rear 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: Striking, Ranged, Technique, and Elemental Resistance, which compromises of the six Elements; Fire, Ice, Wind, Lightning, Light, Dark Resistances. Although, it if common to assume that attacks can be a hybrid, that of which can be Striking and Elemental, the game only takes one type into consideration for the computation in regards to resistances.
Unlike the Rear, 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. Furthermore, Sub Units of high rarity are difficult to come across, with most being accessible through Phantasy Star Online 2es.
Units of or higher can be affixed with a Ring Skill. This process requires a Left Skill Ring, a Lambda Grinder, and 100,000 Meseta as payment. When taken to the Item Lab, the Left Ring can be destroyed and its Ring Skill (at the level equivalent to the Ring's level at the time of affixing) transferred to the Unit; when a Ring Skill is affixed, the player will permanently receive its effect as long as the Unit is being equipped. Ring Skills can be overwritten with other Ring Skills, removing the previous Ring Skill in the process. Ring Skills do not consume Special Ability Slots.