Active Directory supports multi-master replication of the directory data between all DCs in the domain. Some changes are impractical to perform in multi-master fashion, so only one DC, called the operations master, accepts requests for such changes. Because the operations master roles can be moved to other DCs within the domain or forest, these roles are sometimes referred to as Flexible Single Master Operations. In any Active Directory there are five operations master roles. Some roles must appear in every forest. Other roles must appear in every domain in the forest.