In most cases, only purebred animals recorded with a breed registry are eligible to enter and compete.
A judge or team of judges will evaluate individual animals both in terms of the correct conformation for the species and the specific phenotypical characteristics of the given animal's breed.
In some cases, shows for many different breeds of animals may be combined in a single event, as in a dog show, were different classes exist for each breed, or, particularly in the case of livestock shows, competition or multiple species of animals may be combined into a single event, such as a county fair, thus allowing organizers to combine fixed costs for common areas such as barns, show rings and the like.
In the case of horses, horse shows limited to a specific breed are very common, and generally include not only the conformation judging common to all animals, but also add performance classes where the animals are either ridden or shown in harness to display their training and underlying athletic ability, manners and temperament.
In any case, judges need to have a vast amount of knowledge and experience and retain objectivity in spite of any personal preference.