To ensure that clients can locate a domain controller in the nearest available site, domain controllers attempt to register their DNS service location (SRV) resource records. These resource records pertain to sites that contain no domain controller for the domain of which they are a member. This functionality is commonly known as "automatic site coverage."
Automatic site coverage factors in the cost associated with the site links of a site without a domain controller. This cost helps determine which domain controller registers its SRV resource records for that site. The SRV resource records are registered by domain controllers from the site that has the lowest cost between its site link and the site that has no domain controller. This makes it possible for clients in the site without a domain controller to use the least expensive network connection to contact a domain controller in another site.
Source:
http://technet.microsoft.com/en-us/library/cc732322(WS.10).aspx
http://technet.microsoft.com/en-us/library/cc978016.aspx