This post is older than a year. Consider some information might not be accurate anymore.
The TPOS text frame contains the information if the track belongs to a multi set of music cds. Check http://id3.org/id3v2.3.0. The ‘Part of a set’ frame is a numeric string that describes which part of a set the audio came from. The value may be extended with a “/” character and a numeric string containing the total number of parts in the set e.g. “1/2”.
Example: Set disc number 1 of 2 with eyeD3
eyeD3 --set-text-frame=TPOS:"1/2" *.mp3
Example: Set disc number 2 of 2
eyeD3 --set-text-frame=TPOS:"2/2" *.mp3
