The only difference in the categories is the location on the ship. Carnival (and other lines) "thinks" that certain locations are better than others, but it really depends on where you'd rather be!
For instance, we like to be above and below passenger cabins, amidships. If we booked an 8B we wouldn't want an "upgrade" to an 8A if it was below a restaurant or a pool deck. (This is just an example and doesn't have any "real life" application.)
Cabin 8421 might have some noise. It is below a restaurant, but above another passenger cabin.
Hopefully, Carnival has insulated these cabins so that you won't hear chairs scraping!
I posted a question recently about a cabin on the deck below the Lido deck, I was concerned about noise. This is one of the replies that I got..........................
Husband and I were just on the Spirit in May and we were in cabin 8171, the Panorama deck and I would not want this cabin again. You have great views, nothing obstructed your view, but like you said it is directly underneath the Lido deck. We could hear scrapping noises from the chairs being moved away from the tables. This noise was worse very early in the morning and of course at lunch and dinner. After a couple of days, it starts to grate on your nerves or at least it did mine. If you can't change decks, I wouldn't worry about it...but like I said before, I wouldn't book this cabin again.
we in in cabin 8423 on the Conquest and never heard a thing.
Except for a few "bumps", we never felt a thing. I never did figure out what the "bumps" were. Similar to driving thru a short section of gravel. I'd gladly stay in that cabin again, or one in the same general area.