Any songs that are eligible for distribution will show the Distro icon next to them in your library or appear in the list of songs when you go to release a new song from the Distro Hub.
Songs that are eligible for Distro must be at least 30 seconds and either be:
Recorded over a Distro Beat OR
Uploaded to the app
Songs that are recorded over a non-Distro Beat are not eligible without a license or permission from the producer. If you have purchased a license or have permission from the producer to distribute the song, then you can export it as an audio file and re-import it back into your library (use the quick upload on your profile page), and then you can distribute the track.