Unassigned tags appear at the top of the layer list in the transition and behavior designers. When you assign a transition to a new set of screens, you should edit it and assign the tags as desired.

To assign tags, drag them from the unassigned tag area at the top of the layer list, onto the desired layers. In the Transition Designer, tags with a red color are for the incoming screen and tags with a blue color are for the outgoing screen. In the behavior designer, the tags have a purple color.