When a Domain controller creates a security principal object like a group or a user, it attaches a unique SID (Security ID) to the object. The SID contains:
A domain SID, which is the same for all Security IDs created in a domain.
A RID (Relative ID), that is unique for every security principal Security ID created in a domain.
Reference:
https://docs.microsoft.com/en-us/troubleshoot/windows-server/identity/fsmo-roles