This may seem like a dumb question, but how do I populate "Playlist Files on Disk"? If I save a playlist from the right pane using "Export Playlist", it lands in one of my directories, as I specified. It also seems, by an unseen hand, to migrate eventually to Amarok Database. If I click on the Save icon, the playlist shows up in the Amarok Database. But it doesn't show up in Playlist Files on Disk. I do have it checked in Visible Sources.

Playlist files on disk can contain entries from 2 sources:
- playlist files found in your collection folders by the scanner. So if you export to a collection (sub-)folder it will eventually be added.
- when long clicking the save button you get an alternative option to save as a file. These files end up in $KDEHOME/share/apps/amarok/playlists.

The last option is useful if you don't trust the database not to get corrupted. Usually that means you are a developer or git version tester