A multi-disc (or multi-medium) release is any release that includes two or more physical discs or other mediums or, less frequently, a digital release which is marketed as having several "discs". The discs must be sold together to qualify as a multi-disc release. Different volumes in a series that are released separately should stay separated even if they are called "CD 1" and "CD 2", as should UK single pairs that are released with "CD 1" and "CD 2" in their titles.
Each physical medium of a multi-disc release is entered on a different medium inside that release. However, both sides of a vinyl or cassette count as a single physical medium and should be entered as such.
Exceptions can exist, dualdiscs are usually entered as two mediums if the tracklist is the same on both sides, but with different mixes.