Rocks, RIR and BO are one of the parent breeds used in making sex links, the main differance is that when the chicks hatch they will come out with the boys being yellow chicks and the girl chicks will look like a RIR, BR or BO chick. So they can be seperated by color (has to do with girl babies getting thier father's coloring, and vice versa). Since you want only hens, it's your best way of not getting a surprize rooster, that you can get when you order pure breed chicks (mine were supposed to be sexed, but ended up straight run).
*note: very simplified explaination, but for what you're asking for that's the chicken genetics you'll need for today.
*note: very simplified explaination, but for what you're asking for that's the chicken genetics you'll need for today.