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in addition to storing info about a physical item, a MARC record can either link that same description for a digital copy of it (see the records in the catalog for anything in VARIATIONS), or they could describe only a digital item. There's a standing debate out there about how to deal with an item held both analog and digital, whether to have one record describing the physical item but having a URL for the digitized version, or having two separate records.
A FRBR item is often physical, though it may be digital.  In Variations2, the Media object is always/only a digital file.
an extremely small percentage of works (I can find estimates if you want - let me know) actually benefit from FRBR-like treatment, and music is one area where the benefit is concentrated.