How does "Best of Show" work? Is it only the category winners that advance, or the top point getters, or is it competition-specific? How is the judging done anyway, are the beers being compared to each other, or are they being compared to their own styles?
In most competitions the BOS round contains the winners from each category. Organizers are allowed to change that though. Often there is a minimum score required to go to BOS.
In BJCP sanctioned competitions, each beer is scored individually according to how well it meets style and technical requirements. At the end of a flight, judges sometimes re-taste the highest scoring beers to pick the winners for the category. If that is done, then they are being judged against each other, although the style guidelines are still in mind.
In other competitions, anything goes.