dga,
As you know RealTracks are audio files developed by PG Music. The audio files are essentially fragments consolidated together into one audio file. BiaB and RB "knows" what each fragment contains so the programs can extract a fragment and use it intelligently in a song.
PG Music introduced UserTracks in 2014. UserTracks expands on the idea of RealTracks. UserTracks are audio tracks and sgu files developed by users. An audio track is recorded. The user then creates a sgu file that provides a bar by bar explanation of what is recorded in the audio file.
The audio track file and sgu file must share the same name. The UserTrack gets it's name from the name of the folder that contains the matched set of files. For example, you can have an audio track called "A.wav" and sgu file called "A.sgu" and place them in a folder called "C" and the UserTrack can be imported into BiaB or RB and will have the name "C". Later on the user might want to add additional audio so they would record audio file "B.wav" and create sgu file "B.sgu" to give a bar by bar description of the new audio. The "B" pair of files can be added to the "C" folder then both "A" and "B" audio are part of the "C" Usertrack.
BiaB and RB will only look for UserTracks inside the UserTrack sub folder. The sub folder must reside inside the bb\rb\RealTracks\UserTracks sub folder.
UserTracks have some limitations. The most well known is 3/4 time doesn't work but there are more. The great news is Dr. Gannon has committed to fixing the limitations.
You can find out more about UserTracks
++ HERE ++ There is a UserTracks forum
++ HERE ++