Anything But rules are entirely up to the organizer. At most KCBS contests, those entries will be judged by the same judges that score the main categories. That saves the organizer the problem of rounding up enough extra judges to rate just that category.
A good set of reps will also help the organizer with the rules to keep the category fair for the partipipants and manageable for the judges.
What may be submitted is up to the organizer. If it's wide open, Anything But becomes a de facto Dessert category. After already going through several categories of meat, most judges are naturally going to be ready to try something that isn't meat. This is a case where prime rib would likely rate far behind cheesecake, pie, banana pudding, etc.
Presentation rules are also up to the organizer. Some insist the entries must fit within a standard 9x9 clamshell and that box must be closed. Other organizers will have an anything goes philosophy and entries may be submitted in anything from a clamshell to a silver platter.
Before you commit to participating in the Anything But, make sure the organizer is absolutely clear about all that category's rules. If he or she is hazy or tells you those details will be determined later, I would take that uncertainty as a clear signal to skip it.
Hope this helps.