From talking to others, lots of research, and personal experience, I category chickens into three groups - layers, meat birds/dual purpose birds, and broilers.
Layers would be the chickens that don't get big and heavy and are prolific egg layers - like Leghorns.
Meat birds/dual purpose birds are the chickens that do get big and heavy - like Buff Orpingtons. You can harvest them about 14 weeks on, depending on your circumstances. They lay eggs too - well the pullets/hens do.
Broilers are the birds that get heavy really fast - like Cornish Rocks. They can be harvested starting at eight weeks on.
I have all three. I threw away the rule book for the broilers when I became disgusted with how quickly they gained weight and their lack of mobility. I put them with a regular flock (once they could walk again) . The cornies walk, run, jump, climb, fly, and free range. They are no longer short stubby chickens who just eat and poop. They even have feathers covering their whole body now! The cornies are hefty (weight wise) but so much healthier than before. I will be harvesting them when I want to, not when someone else tells me I am suppose to.
I hope this helps you find the answers you are looking for.