These are relevant references from the Books where Plenipotentiary is mentioned.
I make no pronouncements on these matters, but report them as I find them.
Arrive at your own conclusions.
I wish you well,
"Behold Lord Grendel," said Decius Albus to the gathered assembly, indicating Kurik of Victoria, "ambassador and plenipotentiary of the great Ubar, Lord Arcesilaus, of rich Mytilene, amongst the Farther Islands, well beyond Cos and Tyros, come to establish ties of friendship and commerce between that great ubarate and our own glorious Ar."
"Even before our rumor," said Seremides, "before risking the transportation of Talena to Ar, which may be expensive and difficult, it is likely that Lurius will have sent to Ar for a plenipotentiary to confirm the identity of his prisoner, to confer as to the nature of her delivery to Ar, and to arrange for the payment of, and possible delivery of, the reward."