These are my notes for using the Clip (V02.01.16A) in UMS mode (USB mass storage device class). They are mostly gleaned from tinkering and from the Clip forum. Most of this is independent of the host platform and so applies to Windows as well as Linux.
The Clip uses M3U playlists (cf. Sansa FAQ). In compiling the collection of playlists, it apparently searches the MUSIC directory and its sub-directories for files with the m3u extension. The base name of each M3U file found appears as an item under the menu. For example:
#EXTM3U Easy Rider\10 - Ballad of Easy Rider.flac Born in the U.S.A\07 - No Surrender.flac Easy Rider\03 - The Weight.flac Born in the U.S.A\03 - Darlington County.flac
File paths are relative to the directory containing the M3U file. To include an audio file residing in the same directory as the playlist, the playlist simply uses the name of the file, without any path information (e.g. "Born to Be Wild.mp3"). For an audio file in a sub-directory, the playlist must use the relative path prepending the sub-directory to the file name (e.g. "Easy Rider\Born to Be Wild.mp3"). The path separator is the backslash character ("\") of FAT file systems. Each filename in a playlist must specify the file's extension.
Playlists do not support absolute path names. A side effect is that playlists can only refer to songs within the MUSIC directory hierarchy. (Peaking into MTABLSE.SYS suggests that an absolute path looks like mmc:0:\PODCASTS\Radio Times\AlphabetJuice.mp3.)