Visual Studio Team Services now supports triggering a release, in team services release management, when a defined tag is available in a completed build. This helps you to filter a build or builds, to get deployed via the release pipeline, out of builds that are successfully completed.
For example a build that is tagged as RTM, would trigger a release while another build, completed successfully, but with no tag is not triggering a deployment. In order to say a release should only be triggered with a given tag, it should be defined in the Triggers tab of the release definition.
A build that is done will trigger a release only if the tag is added to the build after it completes successfully. When the tag is not specified release is not triggered.
Even if you set the tag after the build completes, no release is triggered.
Only way to get a release triggered, when the build done is to set the tag wile the build is in progress. You can do that by clicking on the build number once it starts. Tag should be set before the build completes to make the trigger work. This is not that cool . Ideally it should trigger a release once tag is set in my opinion.