length is the length of the *0 file in bytes. For tunes, it's the length of the MP3. For playlists, it's the number of children multiplied by 4 (the size of a FID). It's used when parsing the /empeg/var/playlists file.

pickn and pickpercent are the values from the Properties|Playlist tab in emplode.

otions is a typo. It's "options". It essentially encompasses the other options from the same tab. Check out lib/protocol/fids.h in the emptool sources for the info.

duration is the length of the song in milliseconds.

offset is how much of the file to skip to get to the first valid MP3 frame (for skipping ID3 tags/album art/etc.)

skipped_count is probably unused. Otherwise, it does exactly what it says on the tin.

play_last is the last time the song was played, expressed as a Unix time_t.

gfid -- no idea.

skip_count -- it's not different. One of them is a typo (can't remember which right now). Probably introduced by an earlier version of emplode or jemplode.