Would also love some support for a subset of tracks on the same disc as opposed to all tracks on a single disc or all tracks.
Cfr. Add discsubtitle to string options - #6 by matt77hias
Just asking Gemini:
MOVEMENT seems officially supported by MP3 (ID3v2.4) and MP4.
TRACKSUBTITLE is not officially supported.
ID3 Tag Usage
- SETSUBTITLE:
This tag applies to the entire set of discs or tracks. It’s used when you have a multi-disc album or a collection where a subtitle describes the whole package. For example, if you have a “Greatest Hits” album released as a 2-disc set, “Vol. 1” might be the DISCSUBTITLEfor the first disc, while “The Definitive Collection” could be theSETSUBTITLEfor the entire release. - DISCSUBTITLE:
This tag is specific to a single disc within a multi-disc set. It describes all tracks on that particular disc. If a box set has discs with individual themes (e.g., “Disc 1: Ballads,” “Disc 2: Up-Tempo”), those themes would be DISCSUBTITLEvalues. - MOVEMENTNAME:
This tag is designed for a subset of tracks on the same disc that constitute a larger musical work, such as movements in a classical symphony or parts of a suite. For example, in Beethoven’s Symphony No. 5, “Allegro con brio” would be the MOVEMENTNAMEfor the first track that is part of the first movement, and subsequent tracks making up that movement would also share thatMOVEMENTNAME. - SUBTITLE:
This tag is for a single, individual track. It provides a secondary title or additional descriptive information for that specific song. For instance, a track might be titled “My Song,” but its SUBTITLEcould be “(Live at Wembley)” or “(Remix).”
This tag applies to the entire set of discs or tracks. It’s used when you have a multi-disc album or a collection where a subtitle describes the whole package. For example, if you have a “Greatest Hits” album released as a 2-disc set, “Vol. 1” might be the 
This tag is specific to a single disc within a multi-disc set. It describes all tracks on that particular disc. If a box set has discs with individual themes (e.g., “Disc 1: Ballads,” “Disc 2: Up-Tempo”), those themes would be 
This tag is designed for a subset of tracks on the same disc that constitute a larger musical work, such as movements in a classical symphony or parts of a suite. For example, in Beethoven’s Symphony No. 5, “Allegro con brio” would be the 
This tag is for a single, individual track. It provides a secondary title or additional descriptive information for that specific song. For instance, a track might be titled “My Song,” but its