Some federations ( list here ) such as the NPC or a show promoter (the person running the show) will allow you to cross over and compete in different classes within a show. For example, you may be able to compete in figure and bikini divisions or figure and physique in the same show. Other shows or federations do not allow that. You must check all the rules for that federation and specific show before registering. Most federation sites will have a list of shows for the year with the show promoters email so you can contact them ahead of time if need be for planning purposes.