Units (ユニット) are considered to be a group of idols (or idol, in the case of a solo unit) who perform together under a given name. Almost every unit in THE iDOLM@STER franchise has a Producer.
Main Series Games
In the arcade release of THE iDOLM@STER, players could choose one, two, or three idols to produce at one time. This meant that no idols outside of the producer's selected unit could be used on or offstage. In solo units, only one idol would be chosen to be produced, do promotions, and stage promotions. Duet units allowed for two idols to work together at the cost of making the game harder. Three idols could be produced in trio units, but this also added to the difficulty of the managing the unit and its tension. When the game was ported to the Xbox 360, this designation of units was kept.
When Live For You! was released, idols were available to be used onstage in a different way. Idols could perform onstage as a unit, but the unit would not be a permanent unit like the previous game. Instead of the objective of producing an idol unit for as long as possible, the objective of the game was changed to allow players to customize performances with the members and costumes they preferred without the usual timeline. While performances were still limited to one, two, or three idols, players could now switch out their idols whenever they wanted. Temporary, customizable unit stage performances would be available in all later games under the named "Office Mode" (SP), "Stage Mode" (DS)", or "Stage For You!" (2, OFA).
SP and Dearly Stars did away with most of the previous rules of units. Idols could only be produced in solo units, both on and off stage. While they could occasionally have group communication events, they were simply not allowed to be produced in duo or trio units. However, trio units were marketed in game. SP's separation of idols into Perfect Sun, Wandering Star, and Missing Moon grouped the 765 Production idols into a force against 961 Production's Project Fairy. 876 Production's idols were also considered solo units, but were often grouped together as an unofficial unit. Even so, "Office Mode" in SP and "Stage Mode" from DS were still limited to solo performances. Songs that could originally be performed by three idols now only used the solo version of the dance.
THE iDOLM@STER 2 reintroduced some of the original concepts back into the game while including new content. Idols could now only be produced in trio units, but could also perform quintet songs onstage by inviting other idols outside of their trio unit to go onstage with them. While only trio units could be produced, several solo units remained, such as the Dearly Stars trio. As mentioned before, "Stage For You!" allowed for solo, duet, trio, and quintet performances. However, quintet performances were limited to certain songs and stages.
SHINY FESTA once again split the idols into separate groups. The story of the game allowed idols to be produced together, but were now considered two quartet units (Honey Sound and Melodic Disc) and a quintet unit (Funky Note). While the idols could be produced together, stage performances were limited to certain songs and were limited on a song by song basis to solo, duet, trio, quartet, or quintet performances with no customization available. However, a few songs allowed all 13 of the 765 Production idols to stand onstage together. This was later expanded upon when the game was ported to the PlayStation 3 via Shiny TV. These units would be called ALLSTAR units in later games.
ONE FOR ALL expanded the scope of idol producing again by allowing players to work with all of the 765 Production idols together. Producers would be managing them in groups of one, two, three or five. "Stage For You!" returned in full and the system of using one, two, three, or five idols onstage was also used. ALLSTAR units (using all 13 cast members) were also made available for certain encore songs via by defeating specified in game challenges.
Units seem to have a fundamental difference when compared to the main series games. While idols could only ever be part of a single unit per storyline in the original games, idols in the mobile games are able to perform in multiple units.
In the Cinderella Girls idols can be produced in groups of five. Several in game units come with preset names, conversations, and events available. However, each unit can easily swap out members and multiple versions of the same idol are allowed in the same unit. Interestingly, units seem to be based on event attendance and appear to be rather temporary. An example would be how Uzuki Shimamura is part of the "New Generations" unit (in multiple aspects of the franchise), part of the unit "Pinky Cute" in the "1st Dream LIVE Festival", and can also be in an unrelated unit created by the Producer.
Million Live! has a different way of looking at units. While units are normally comprised of three idols, the girls were put into groups of five as official units, such as Legend Days. Even still, the newer idols can be produced alongside the original 13 765 Production idols.
Side M handles units in an almost identical way to Million Live!, with the exception being that the boys were introduced in preset units. The idols can be produced outside of their preset units like Million Live! and are available alongside the reformed members of Jupiter.
Types of units by game (Table)
For a list of official units see the Official Unit Listing.