It’s very common that box sets of the label Disky (typically compilations) have discs (often jewel cases) with own catalog number and barcode. The box set usually has it’s own catalog number and barcode.
It’s not clear at all they also have been released and distributed separately or only as box set.
I think, they have been released only as box set.
How to handle it?
AFAIK it’s not possible to link mediums of releases into another release (only “RG includes/included in” relationship).
It’s just possible to “copy” a release medium to another release. It’s easy to do but with the disadvantage to have the data twice.
Since I can not say it definitely that the releases have not been releases separately, I just can make a new release group/release for the box set with copied data.
Discogs has it as separate releases and as box set. Musik-sammler.de only has it as box set.
I know other topics about box sets and merges. The problem is not how to do it technically, the question is what is correct and the best way here.