If you setup your TFS build server to delete old builds you might not know TFS deletes the label for that build as well. If you one of those old builds you want to have the label you used to build it. Many ways to deal with this issue. Changing the AT web.config is how to change it for all builds. The builds will be deleted, but the labels will remain.

From: http://social.msdn.microsoft.com/forums/en-US/tfsbuild/thread/a08bf751-4...

"The build admin can add a flag to the Web.config file on the Application Tier to specify that labels should not be deleted. Here is an example of what that line in the web.config file would look like:

<add key="PreserveLabelsOnBuildDeletion" value="True"/>
"