The Pater Patratus was the title of one of the priests known as fetiales, whose duties concerned treaties with other cities. Nonius Marcellus quotes a passage from Varro, De vita populi Romana, book 3, concerning the fetiales.
FAETIALES apud veteres Romanos erant, qui sancto legatorum officio ab his, qui adversum populum Romanum vi aut rapinis aut injuriis hostili mente conmoverant, pignera facto foedere iure repetebant; nec bella indicebantur, quae tamen pia vocabant, priusquam quid fuisset faetialibus denuntiatum. Varro de Vita Populi Romani lib. II.: ‘itaque bella et tarde et magna diligentia suscipiebant, quod bellum nullum nisi pium putabant geri oportere: priusquam indicerent bellum is, a quibus injurias factas sciebant, faetiales legatos res repetitum mittebant quattuor, quos oratores vocabant.’ — idem lib. III: ‘si cuius legati violati essent, qui id fecissent, quamvis nobiles essent, uti dederentur civitati statuerunt; fetialesque viginti, qui de his rebus cognoscerent, iudicarent et statuerent et constituerent.’
The FETIALS were those among the ancient Romans who, being in the holy office of envoys, demanded, from those who had started a war against the Roman people by force or robbery or the insults of a hostile mind, once an agreement had been made, a treaty by law; nor were wars declared, which were called justified, before a declaration had been made by the fetials. Varro, On the life of the Roman People, book 2: “And so wars were undertaken slowly and with great deliberation, because they thought it wrong to wage any war unless it was justified. Before they declared war, they sent four fetiales as ambassadors to make a claim to him, by whom they knew that the injuries had been committed, and they called these ‘orators’.” — likewise book 3: “If someone else’s envoys had been outraged, those who did it, even if they were noblemen, were held (?) so that they might be handed over to the [foreign] community. And twenty fetiales, who are learned in these matters, judged, decided and legislated.”
The Pater Patratus was selected from these four, I have read somewhere.
Again, this gives us a little more background information.
-  There is a discussion in Robert E. A. Palmer, The archaic community of the Romans, p.186. ↩
-  Nonius, p.529 M = 850 L. The numerals are the page or column numbers of an ancient edition, reprinted for reference in the margins of the more modern editions. The passage may be found in vol.3 of the W. M. Lindsay Teubner edition, on p.850. ↩
-  Lit. “pious”. ↩