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