They function the same as any RealTrack once they are opened in the mixer. They respond to the chord chart, tempo and key signature the same as RealTracks.
They do have to be saved into a specific folder bb\realtracks\usertracks
There is also a specific naming process but the creator of the User Tracks you download would have taken care of that task.
So, download the file and place it into the UserTracks folder, from the mixer, right click and select Select a User Track for this track and it should perform the same in your song as a RealTrack.
This link describes how to create, save, load and use a simple User Track.
Band-in-a-Box: Make a Pop UserTrack in less than 10 minutes!