TPOS
The 'Part of a set' frame is a numeric string that describes which part of a set the audio came from. This frame is used if the source described in the "TALB" frame is divided into several mediums, e.g. a double CD. 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".
Source:
https://id3.org/id3v2.4.0-frames
In short, same purpose as the fild contained in the Vorbis scheme and would benefit a lot when rendering to MP3 as a destination format for digital distribution in such case.