TFS Release Management allows you to use deployment metadata as a value for configuration variables in release templates. It comes in very handy when you need to refer to the build drop location or build number. The following is the list of available Metadata that can be used with Configuration Variables.
Build Directory $(PackageLocation)
Build Number $(BuildNumber)
Build Definition $(BuildDefinition)
TFS Collection $(TfsUrlWithCollection)
Team Project $(TeamProject)
Unfortunately, you cannot create custom deployment metadata just yet. Hopefully that will change one day. Another "catch" is that deployment metadata can only be used in components. Deployment metadata cannot be used in actions or tools because those are taking place outside of the build process.