Relations are a super-class of table-things. There are many subtypes of relations. \d Lists relations of all subtypes. You can also add a single letter to \d to limit the listing to only the subtype-specific relations. That is what the E is doing here.
That's an obtuse answer. If everything is "a table thing", why even bother printing a title? Normally, select() doesn't show a title, so if you're just going to dump data to the screen with misleading titles, why bother with a garbage title? And how is \di, list indexes "table things"? You can't select from an index, so for users they're not relations.
It seems to be another list of foreign tables but with a different format.
Since \d can only show information limited to the super-class relation, and not specific subtypes, the information for the foreign table subtype needs to be made available somewhere. That place is \det
I don't know what that means, but when two menu items have the same text, but a different function, Houston, we have a problem.
I also kind of feel like the name in the help text should always be the name you actually get. So for example, \ddS is described in help as "show object descriptions not displayed elsewhere" (BTW, why is this one "show" and the others are "list"), whereas the output is merely "object descriptions".
This kind of review of consistency doesn't really get treated like a bug report in most cases. It makes for a decent technical writing R&D project for someone interested in putting in the effort.
It's not technical writing, it's part of the program. As much a part of the program as any menu in a gui program, or window title in a gui program.
