The Paternova is the head of the entire Navigator lineage, with his Envoy representative the actual High Lord attendant. Novators are the matriarchs and patriarchs of each house, with the Heir-Apparent representing the most knowledgeable and powerful (and often eldest) of their ranks, who will compete to become the next Paternova upon his death. Navigator Primus' are the senior Navigators, with Navigator Scions representing their groomed public faces to the courts of the Imperium.
So that's how these are set up. I always wondered. I get the Paternova is the current house head, and the heir self explanatory, but what do the rest mean?