This is a special identifier that can be assigned to one or more Primary AuthIDs. The privileges granted to the GroupID are available to every Primary AuthID to which the GroupID is assigned. In addition, each Primary AuthID can have multiple GroupIDs assigned to it, thus gaining the privileges of all of those groups combined. Other privileges can be granted to the Primary AuthID, providing the user with the assigned group privileges plus the privileges assigned specifically to the Primary AuthID.
If you specify a GroupID as the second part of an object's three-part name, all users who are assigned to that group will automatically have all privileges on the object. Privileges cannot be revoked from a user whose GroupID is the same as the object creator's AuthID. The GRANT and REVOKE commands have no effect on these privileges.