TBH, as a former mod, I usually lump everything under Marvel with the exception of X-Men (just because it has such a HUGE group of characters and its own fairly large movie franchise separate from the MCU stuff). I don't bother with MCU at all, because then you have to have a separate category for stuff like the animated and comic book-specific stuff and I'm just...way too lazy lol
But, as I understand it, usually MCU is reserved for I think characters that were in The Avengers and the movies from whence they came [ie: Thor, Iron Man (1,2,3), The Incredible Hulk (2008), and Captain America].