COOP is the registrant because the aquired sub (formerly Nationstar) is doing the main business. This sub (Nationstar Mortgage LLC, d/b/a Mr. Cooper) does the whole mortgage servicing business, but the parent Mr. Cooper Group, Inc. (COOP) registered the shares. We are shareholders of COOP, not the sub, hence COOP is the registrant. A holding company.
Before Nationstar was acquired by WMIH, Nationstar registered its shares directly (NSM), so Nationstar was the registrant.