It applies to any chickens you butcher when they're quite young--8 weeks old or so.
That's when Cornish Cross are often butchered: at that age, they're bigger than most breeds are at full maturity! If you try to keep them longer, they often start to die of "natural causes" (heart failure, too heavy to move from feeder to waterer and thus die of thirst, etc.)
Any other breed of chicken grows much more slowly. You can butcher them at 8 weeks (or even younger, if you wish.) But people often do raise them for much longer, which means they're more mature, which means they can be treated like "chickens" instead of like "baby chicks."
The reason for raising them longer is that most people prefer to butcher bigger birds, so they don't need to do as many of them. On butchering day, "one bird" takes about the same amount of time, no matter what size it is.) It's quite possible to raise tiny little birds (quail, pigeons, or bantam chickens), and butcher enough of them to fill a freezer--but it's often easier to raise big birds (turkeys or cornish cross).
In the summertime, any kind of chicken can do fine with a pen that has a roof, a bit of protection from wind/rain in one corner or end of the pen, and hardware cloth sides on the rest of it. They'll prefer to have roosts for nighttime once they're a few weeks old (Cornish Cross get too heavy to roost, so don't bother with roosts for them.)