A Cluster (クラスター, kurasutā) is a unit of 437 Ex-Machina. There were 32 clusters during the end of the Great War, including the Emir Cluster led by Emir-Eins and the Über Cluster which Schwi formerly belonged to. Ultimately, all clusters follow their call-sign Einzig.
A cluster is able to forcefully excommunicate a unit from the rest of the cluster. This can be due to many reasons, such as the unit experiencing errors (like when Schwi wanted to know what a heart was). An excommunicated unit may also seek permission to reconnect with the cluster in special cases. However also in Schwi's case, it took 251 seconds to sync all data with the cluster due to the large amount of data associated with her "heart" (she mentioned that there were no amount of data required more than 3 seconds to sync before). During this, total damage to the unit is unacceptable.
Also called "The Collective" when spoken to, it acts as a hive mind able to alter any Ex-Machina to whatever it sees fit. Its ultimate goal is to become a collective of information in order to advance themselves even further.