Tech Talk

Flexible Single Master Operations (FSMO, sometimes pronounced "fizz-mo") roles are also known as operations master roles. Although the AD domain controllers operate in a multi-master model, i.e. updates can occur in multiple places at once, there are several roles that are necessarily single instance:

Role Name Scope Description
Schema Master 1 per forest Controls and handles updates/modifications to the Active Directory schema.
Domain Naming Master 1 per forest Controls the addition and removal of domains from the forest if present in root domain
PDC Emulator 1 per domain Provides backwards compatibility for NT4 clients for PDC operations (like password changes). The PDCs also run domain specific processes such as the Security Descriptor Propagator (SDPROP), and is the master time server within the domain.
RID Master 1 per domain Allocates pools of unique identifier to domain controllers for use when creating objects
Infrastructure Master 1 per domain Synchronizes cross-domain group membership changes. The infrastructure master cannot run on a global catalog server (GCS)(unless all DCs are also GCs.)