Disc titles are the titles sometimes given to individual discs (tapes, etc.) in a multi-disc release.
Note that the disc title is not the main title:
- The main title is the title that applies to the whole release.
- The disc title is the "sub"-title that apply to one disc only (to differentiate them)
The disc number style should be used for these titles.
"Mellon Collie and the Infinite Sadness (disc 2: Twilight to Starlight)"
"Twilight to Starlight" is the disc title in this case, while "Mellon Collie and the Infinite Sadness" is the main title.