Stores all configuration data for the currently logged on user. This is a db view of HKEY_USERS\SID_of_currently_logged_on_user. As a db view, if an change is made in one, it is reflected in the other. The subtree pairs are not copies but one is a db view of the other. Its the same db structure. The HKEY_CURRENT_USER is just easier to find for users and developers than a subtree with a SID number involved.