The only requirement is that they support the OpenDocuments apple event. You may open multiple source folders if you choose. You can also double-click files to add to the playlist.
File names turn red in color to indicate they have been added to the playlist.
As you build your playlist, you may edit it as needed. The up arrow button shifts the selection up one in the list, and the down arrow button shifts the selection down one in the list. The funny looking off-set rectangles button shuffles the playlist randomly. If the file is stored on a writable volume, you can optionally fill in the ID3 tag information. Trax will then launch your player and send the list of files to play.