In the context of the Microsoft

Microsoft

Microsoft Corporation is an American public multinational corporation headquartered in Redmond, Washington, USA that develops, manufactures, licenses, and supports a wide range of products and services predominantly related to computing through its various product divisions...

 Windows NT

Windows NT

Windows NT is a family of operating systems produced by Microsoft, the first version of which was released in July 1993. It was a powerful high-level-language-based, processor-independent, multiprocessing, multiuser operating system with features comparable to Unix. It was intended to complement...

 line of computer

Computer

A computer is a programmable machine designed to sequentially and automatically carry out a sequence of arithmetic or logical operations. The particular sequence of operations can be changed readily, allowing the computer to solve more than one kind of problem...

 operating systems, the relative identifier (RID) is the part of a Security Identifier

Security Identifier

In the context of the Microsoft Windows NT line of operating systems, a Security Identifier is a unique name which is assigned by a Windows Domain controller during the log on process that is used to identify a subject, such as a user or a group of users in a network of NT/2000...

 (SID) that uniquely identifies an account or group within a domain. The Relative ID Master allocates security RIDs to DCs to assign to new AD security principals (users, groups or computer objects). It also manages objects moving between domains.

The Relative ID Master is one role of the Flexible single master operation

Flexible single master operation

Flexible Single Master Operations , or just single master operation or operations master, is a feature of Microsoft's Active Directory...

for assigning RID.