I defined the content type of the set of documents "A" to contain the metadata of the custom site (columns) 1, 2 and 3. I defined the content type of the set of documents "B" to contain the metadata of the custom site 4 , 5 and 6. I created a document library and configured to include instances of A and B along with the default content types Document, Image, Video and a couple more.
When I'm in Document Set A and upload a document, the metadata assignment screen shows me 1-3 as I expected, but it also shows 4-6 that they do not apply to A; Similarly, when I'm in B and upload a document, the assignment screen shows me 1-3 (which I do not want) along with 4-6. I can not figure out how to restrict the metadata entry screen to only those columns defined within the content type.
If relevant, I had another problem to solve: as shown in the Library Settings window, the default content types were not "Used in" the document sets created from A and B, even though I included the default types within A and B. As such, 1-6 did not appear in the metadata assignment screen when loading. Therefore, in Library Settings, I selected a default type (for example, "Document") and manually added the metadata 1-6 of my custom site and that solved the problem. I do not think it's related to my current problem, but I thought I'd mention it just in case.
It's easy to get lost in the language … I hope this makes sense.