Once parts have been assigned, any user can select a song and will be be able to choose which part they want to view. Once you tap on a song, you will be prompted to select a part from all available parts. Once the part has been selected and downloaded, the file remains available for offline access.
To switch parts:
Navigate back to the Home Screen
Tap on the ⋯ next to the song name and tap 'switch part'
This new part will now be available offline and will be the default part chosen for that user each time the song is opened.