By default when editing parameters inside a Team Foundation Build process template you get a very simple editor.(Except for some out-of-the-box available parameters). But wouldn’t it be nice if you could give your users a much better experience by providing a custom editor?
TFS allows you to this by changing the Editor Parameter in the Metadata argument.
You could build your own editor or you could start by using one of the built-in ones(available in the Microsoft.TeamFoundation.Build.Controls assembly). So just enter Microsoft.TeamFoundation.Build.Controls.{ControlName}, Microsoft.TeamFoundation.Build.Controls in the Editor parameter and everything is set.