Let's say that you have SiteA, SiteB, and SiteC that correspond to the 10.10.1.0/24, 10.10.2.0/24, and 10.10.3.0/24 subnets, respectively. If you delete the 10.10.3.0/24 subnet, any clients using that IP range will randomly select a DC from one of the other two sites by default. Now let's say that you configure SiteA to contain two subnets: 10.10.0.0/16, and 10.10.1.0/24. As long as SiteC is associated with the 10.10.3.0/24 subnet, any clients in the .3 subnet will authenticate against SiteC. But if you delete the .3 subnet, those clients will now use the DCs in SiteA, since they fall under the "umbrella" of the 10.10.0.0/16 supernet.
This was first published in October 2007