I don't think it's about "breeds" as such, not in the KC meaning of the word anyway. I think there are types of terrier about which display certain characteristics, which, maybe not in every instance, define a breed, and I'm using the word "breed" in a very loose, informal way here. When people talk of russells we tend to think of a type with certain characteristics, and there will always be exceptions of course, but on the whole people think of jacks and transmit certain working styles on to them. Same with blacks (or whatever you want to call them), borders etc etc.
What I think Foxdropper