The acronym for Flexible Single Master Operations is FSMO.
Suppose that the organization set more domain controllers. Each found a copy of the Active Directory database copies are synchronized with each other via a data synchronization method of multi-master, in which data is stored in the group, and domain controllers where each controller can update this information. So, I repeat, this is way more main servers or in English Multi-Master Mode. The last entry in the DC base is one that is taken into account in checking the authenticity of the record – time is essential.
Operations which do not correspond to the description above, are thus so-called Single-Master Operation, which means that certain operations only supported on a single server, since data are essential for the operation of data infrastructure AD.