I never fully understood "tweaking" a playlist

There are some things to understand here:

A FID. FID stands for File IDentifier. It is a 32-bit number. The low four bits are reserved so, in effect, you can have 2^28 tunes or playlists on your player. These are the files stored in /drive0/fids and /drive1/fids -- /empeg/fids0 and /empeg/fids1 are aliases for these.

FIDs are expressed in hex. This is convenient, because the low four bits of the ID can be expressed as a single hex character.

Each FID is (generally) stored as two separate files. So, for example, a tune might be stored in 2fdc0 and 2fdc1. We generally refer to this as FID 2fdc0. The *0 file contains the MP3 (or FLAC or whatever) data for the tune and the *1 file contains the tags for the file.

Another documented *whatever number is 'F'. The dynamic data for our example FID is referenced by 2fdcf. This never appears in a file anywhere -- it goes onto the dynamic data partition -- it's just a convenient way for emplode and JEmplode to tell the empeg which dynamic data to write to.

The other documented "subfid" (I just made this term up) is the *2 file, which is used for storing the low bitrate version of a file on the Rio Central.

A playlist is also stored as 2 separate files. Again the *1 file contains the tags for the playlist, but this time the *0 file contains a list of the children of that playlist. These can be other playlists, or they can be tunes.

If a tune appears in multiple playlists, its FID will be listed in the *0 file for each of those playlists. It will not appear on the hard disk more than once.

There's some more explanation of this here.

There are other special FIDs. The root playlist is always stored as FID 100 (so it'll be in /drive0/fids/100 and /drive0/fids/101. The "Unattached Items" (now passed into folklore) playlist is always stored as FID 110. Other FIDs below 100 are used for other purposes. See lib/protocol/fids.h in the emptool sources for a list of these.