Support Unit
In Arknights, the player can assign up to three of their Operators as a Support Unit which can be used by their friends; only one Support Unit can be used at a time, which is counted as the 13th Operator on the squad. This can be useful for players who do not have a particular Operator which is ideal to clear certain operations, especially difficult ones, or simply allows players to try out a unit they do not own, or have not raised.
Initially the player has only one Support Unit slot, which can be increased by upgrading the Reception Room on their base. The Operator in the first slot must be 5★ or below, while the second and third can be of any rarity including 6★ Operators.
Using a friend's Support Unit will give 30 
while 20 will be received by the owner of the Support Unit being used by a friend. Both are included in the daily 
Credit claim and only applies once a day.
The Support Unit Operator's level will be adjusted to the highest promotion of the Operator in the squad, which means that if the player has only Elite 1 Operators in the squad, the Support Units will be limited in at most Elite 1 maximum level regardless of whether the Operator set by their owner has reached Elite 2, and their skills/talents will be downgraded accordingly. As a result, if one uses a Support Unit, it is highly recommended to bring an Elite 2 Operator to benefit from the Support Unit's promotion, even if they ended up being unused. It is worth mentioning that the Operator's level is unaffected; for instance, if the player's highest level Operator is Elite 2 Level 60, they will still be able to borrow an Elite 2 Level 90 6★ Operator and benefit from their E2 L90 stats.
The player can fine-tune the Support Unit selection by narrowing them to the specified class and refresh the list of Operators as many times as they want, but with a cooldown that increases the more the list is refreshed within 10 minutes. The player can also use Support Units of those who are not the player's friends, distinguished by the blue "Recommended" mention on their card, but their skill will be in at most Level 7; friends' Support Units do not have this restriction. Clearing an operation with the Recommended Support Unit gives the player the option to add the owner as their friend, which can also be done in the Support Unit menu.
Old
The reworked system introduced in A Walk in the Dust is a significant improvement over the previous Support Unit system.
Among its many problems, in the old Support Unit system, duplicates of a single Operator could appear simultaneously on the list, refreshing the pool was limited to 10 times every 12 hours (5 times every 6 hours prior to Code of Brawl), and prior to CB, the Support Unit's level was capped at the highest (actual, not Promotion) level of the player's Operators in the squad. The Recommended Support Units were separated from those of the player's friends in their own section, with their skill automatically set to the first skill regardless of whether the owner had set the Operator to its second or third skill; and it would always be at Skill Rank 1, which the game referred to as having "Skills Disabled". Due to how reliant Operators actually are on their skills, this made the previous non-friend Support Units all but worthless.
Trivia
- Under the Contingency Contract "Objective: Covert Action" contract and certain operations' Challenge Modes conditions which reduce the squad size, the Support Unit will be accounted for in the reduced squad size, so the player cannot use a Support Unit if all of the squad slots normally reserved for their own Operators are used.
- In other words, while the default squad size is 13 including the Support Unit, reducing it to 6 will limit it to either 6 of the player's own Operators, or 5 of their own Operators and the Support Unit.
- From C.C. Barrenland onwards, the highest Contingency Level/Risk achieved featuring a unit on the current season's Fixed Site will be shown as they appear on the Support Unit selection, and detailed stats are available in the daily Credit claim for the duration of the season.