Because some people guestioned my argument that Russians saw Veliky Kniaz as a king, I give you actual proof. In 1323 Veliky Kniaz Jurij Danilovitsch and Swedish king Magnus Eriksson made treaty of Nöteborg (Orekhovo).
Whole treaty
As you can see Veliky Kniaz = Rex Magnus
Treaty of Nöteborg said:Ego rex magnus Jwrghe cum borgræwio Alfarmio, cum duce Abraham, cum tota communitate Nogardie, ego terminaueram cum fratre meo rege Swechie, Magno Erici filio. [...]
Dedit rex magnus Jwrge cum omnibus de Nogardia pro amicicia tria gislalagh: Sawolax, Jæskis et Ægrepæ, Carelsk gislalagh.
Whole treaty
As you can see Veliky Kniaz = Rex Magnus