If you have multiple domain controllers (dc) within a site and you would like to have one of these dc's refered to more often or only if no other dc is available. Selection of a dc within a site is controlled by both the weight and priority.
Weight of a Domain Controller
By default all dc's have a weight of 100, the heavier the weight the more often the dc is referred to. The formula for the referral is based is an elementary math. If there are two servers in a site and one has a weight of 100 and the other 200. The dc with twice the weight will receieve twice the referals.
To modify the weight of a dc the registry key, use:
Priority of a Domain Controller
By default all dc's have a priority of 0, the lower the priority the first in priority. The dc with the lowest priority in the site will receive ALL authentication requests unless it is unavailable. If the lowest priority dc is unavailable then the next lowest dc in the site will receive all requests, etc…
To modify the priority of a dc the registry key, use:
For additional information see: http://technet.microsoft.com/en-us/library/cc787370(WS.10).aspx