In terms of the SOP, like with any animal, there is a guide on what it should look like. Every little detail, from eye color to how the bill ties into the head, to how balanced the body is from front to back to how the wings set on the bird and everything in between. At a show, the duck that best represents those traits for the breed as a total package is the winner. When breeding, first you need breeding goals. Then you select the ducks that best represent your goals. Each generation, you breed only those who best reflect what you're looking for.
The difference between pet quality, breeder quality, and show quality is basically how the bird is compared to how it should be. Breeding show quality to show quality helps but you need to know your breed and what you should be breeding for to reach certain goals. Like with call ducks... that short little nub bill gives hatching a difficulty. So you take your best show drake and breed him to a female that has a slightly longer bill. If you keep breeding the bill smaller and rounder each time, eventually you'll end up with ducklings that can't hatch themselves. So a you need breeder quality duck for breeding, to potentially make show ducks. The duck herself may not show well, but her offspring should. Thus making her breeder quality.
A pet quality call duck, will have a bill as long as a mallard and some other flaws that would eliminate it from showing, and it will take several generations to improve on the type, so the pet quality duck can't really be called breeder quality since it's so far removed from where it should be in terms of the SOP.
Then to make it even more fun, it's all open to interpretation and personal opinion. What wins at one show can lose at another, and someone's $40 pair of breeder quality ducks might be $10 pet quality culls to someone else.