"Extra" content limit on maps and what counts

I never knew about that before, but there are claims that maps have a maximum amount of "extra content".
So what exactly counts toward it? For example, does "core content" (beasts, betrayal, delve and incursions) count? If so i would absolutely avoid specking into beasts, because "extra" content (that can be blocked) is X100 more worth than some extra beasts, but my main questions is - harbingers and beyond demons (and stronboxes(lol???)) counts as well? If so, I would not spec them EVER, I always thought there were more some extra risk/some extra reward, but if the take extra content "slot" then absolutely NO

So anyone can 100% confirm?

Have you checked this ?https://www.poewiki.net/wiki/Extra_Content
I doubt it's 100%. Perhaps even as low as 99% but that's essentially how people play the game.