The RID master allocates pool of relative IDs to each DC in its domain. Whenever a DC creates a user, group, or computer object, it assigns a unique security ID to that object. The security ID consists of a domain security ID (that is the same for all security IDs created in the domain), and a relative ID that is unique for each security ID created in the domain. To move an object between domains (using Movetree.exe), you must initiate the move on the DC acting as the relative ID master of the domain that currently contains the object.