Breeder stock is merely birds used for propigatation. In the case of exhibition birds, breeder quality is typically considered intermediate between pet quality and show quality birds. However, some breed standards are such that the best parents are not necessarily the best show birds, so you may actually get better and more offspring from breeder quality birds than frmo show quality. Birds such as cochins and silkies with very short backs and very rounded breasts are good examples of ones that may be better in the show cage than in the breeding pen. Taller birds with less rounded chests tend to breed better, but not fare as well in the show cage.
Heritage stock are bred with the goal of keeping and increasing the population of breeds that were developed sometime in the past and are viewed as being historically significant. The goal is usually to get birds closer to the original standard, while maintaining or improving the production quality intended for the breed. Exhibition birds are often lacking in egg production. Hatchery quality "production" breeds often lack size, which is important for a meat or dual purpose bird, or broodiness, which is important for continuing the breed.
