Using Active Directory Users and Computers snap-in, you can look at the properties of a user object. On the Member Of tab, there is a “Set Primary Group” setting. The “Set Primary Group” button on the MemberOf tab alters the AD “primary-group-id” attribute. The primary-group-id attribute was added to AD to support products like Services for Mac. Not all resources support a primary-group-id setting.
When Identity Management for Unix (IdMU) has been installed, it add a Unix Attribute tab. On the Unix Attribute tab, you can see a “Primary Group Name/GID” setting. The “Primary Group Name/GID” combo-box on the Unix Attributes tab modifies the Active Directory gidNumber attribute, which is created in the Active Directory schema as part of the idMU installation process. The “Primary Group name/GID” was first introduced by Service for Unix (SFU) and is only used by the SFU components. On newer versions of Windows Server, the SFU components have been broken out into separate packages such as Identity Management for Unix (IdMU), Services for NFS, and Services for Unix based Applications (SUA).