These are birds with minor faults just as improper toe spacing, a missing toenail, or even something like a bent toe due to a hatching issue. These are birds that will not do as well in the show ring but it won't hurt them to use from a breeding standpoint. It can be something as simple as that 5th toenail not pointing backwards instead of straight out. There are some things that will definitely pass on that you don't want to be using... 4 toers, single combs, 6th claws, red skin, etc. Some things you can look at as chicks and others first show up at a much later stage.... improper wing placement, off colors in hackles/saddle, horns off the comb, etc. There's also alot of reasons your show birds don't always make the best breeding stock too. We strive for such huge crests, cusions, the S shaped bodies, short legs, little combs, etc. Alot of those same factors impairs fertility and they physically cannot mate anymore without the use of artificial insemination. Its all a balancing act and you have to do alot of matings just to see what works. Its not as easy as just throwing 2 birds together. If one is weak in one area, you balance them with a bird strong in that area. Even the best looking bird you may ever find can still throw quirks in its offspring. You have to learn how to watch for that kind of thing.
There are alot of silkie 'breeders' out there and if you want to sell birds to anything but the Asians for meat, you have to get on the same playing ground as for quality. Silkies are a challenge and it takes alot of dedication. I'm sure it is no different with other breeds too. You mentioned wanting to become a breeder and making money off them. Well its going to involve doing some homework.