Behind this is former service as a U.S. senator (only giving the senator seniority within his or her new incoming class), service as U.S. Vice President, a House member, a cabinet secretary, a state governor, and then by their state's population, respectively.
This background includes previous roles such as U.S. senator (gaining only seniority with the new class), U.S. Vice President, member of the House, cabinet official, state governor, and finally, based on their state’s population, respectively.