I think usually there's a long list which is voted upon and then it's narrowed down to the shortlist, where there's four nominations in each category. I'm not entirely sure but I would imagine most, if not all potential nominees in every single category are initially eligible to be voted for (so BB would be included in the Reality category), but if they don't receive enough votes, they don't make the shortlist. Big Brother always used to, but sadly as the support for the show has declined, it doesn't seem to anymore.
I remember the days where Big Brother and Celebrity Big Brother were both nominated in exactly the same category, essentially making up half of the nominees
