I agree that "heritage" has to do with sustainability, but, to me, it also has a sense of the historic. The SPPA, for instance, tends to focus on this aspect of it.
Also, a lot of people do use the term "heritage" to refer specifically to American heritage breeds, because it offers a tidy way to differentiate these historic birds from other "rare" types. I'm not saying it has to be defined this narrowly, only that this is one of the more common usages.
Also, a lot of people do use the term "heritage" to refer specifically to American heritage breeds, because it offers a tidy way to differentiate these historic birds from other "rare" types. I'm not saying it has to be defined this narrowly, only that this is one of the more common usages.