In answer to this question about the add-on paragraph at the end of 12(b):
That comes up in downstream additional claims (for example, an answer to a counterclaim in response to a third-party complaint or an answer to a counterclaim in response to an intervention). These do not necessarily come within the list of required pleadings under 7(a), although courts allow them. This additional paragraph is understood to preserve these defenses.
We will see more of this when we get to Responsive Pleadings in a few weeks.