These CDs are routinely added as separate records (Discogs, etc.). Over time, these sets are usually broken up and the parts sold as separates; some discs may also be available for purchase as separates upon release. All these would be potentially linked records, links that couldn't be added to sets .
I have a number of discs from sets (in several formats) for which I have just one part. The part I have is the only part I can enter, especially if no information is available for the set.
A good example of set/separates is one of the Marx Brothers DVD sets in the DVD World. The box set contains discs which are also issued separately, in various editions including foreign translations. The box set might seem to be a separate product, but, seeing the linked releases, one can see the situation more clearly.
I also have a number of pop music CD box sets which consist of previously released individual albums. Both the individual albums and the box sets are/will be input.
Some sets are so complex that I haven't added them, and probably never will. Of these sets, some have individual parts that can be input relatively easily; others have sequences that don't work well as individual discs, and could only be input as box sets.
Some discs I have added to 45Worlds seem to be parts of sets, but there is no information on the set. (Sometimes relationships sometimes become clear over time).
Of course, these albums could just be added to Discogs, a major loss to 45Worlds. (So far, I'm not working in Discogs)
Hi Jaime, are these CD's from the 3 Mercury Living Presence boxes available singly? If not, they should probably only be entered with the box sets, however painful that might be! They may be available separately on download, though - I have just downloaded The Romeros' WORLD OF FLAMENCO from Aussie itunes, and it has a 10-page digital booklet of notes, which is a nice touch.