Rename animation tags
You can name tags to make their purpose more clear. For example, if you have a layer called "Untitled-1" and you move it in the Transition Designer, it will become tagged with "Untitled-1". You can rename the tag just like renaming a layer, double click the tag and type a new, more descriptive name
The next time you try to use that transition, it will be much easier to assign the tag to the appropriate layer.