I have a custom content type based on the Document Set content type. I have several custom columns added to this content type. For some reason one of them doesn't show up in the list of Shared Columns within the Document Set settings. This particular column is of type Choice. I'm able to manually add other Choice types of columns to the content type and they show up. What is the exact criteria for a column showing up in the list of Shared Columns within the Document Set settings?
In my case the column definition is (changed names since it should be irrelevant):