The description for marketed is: “This indicates the organization that markets a release. This is not the same concept as the record label.” and for promoted: “This indicates the organization that promotes (or contracts out promotion) for a release. This is not the same concept as the record label.” In my mind that’s exactly the same. Well, except that one of them seemingly can’t be delegated to another company.
I seem to remember it’s mostly a case of “they might or might not be different, but it’s hard to know, so we have both and use whatever applies”. When “marketed” was added, there was some talk of having it be one “marketed/promoted” relationship, but some people disagreed and thought it was a separate thing, so we kept them separate.
Well, that’s a bit odd… So it’s mostly a case of applying whatever is printed on the release itself. It just looks a bit weird when you see those relationships side by side (especially with those descriptions).