In short, it mainly concerns how to translate the Slavic ruler title kniaz' in historiographical tradition (research history).
In fact, some (rather the majority of?) western sources call the ruler of the Rus' before the 13th century (so-called the Tatar Yoke) rather as a "king" (rex in Latin) than as a "duke" (dux) (as they call the ruler of the Poles).
The following passages are examples of such uses of "king" rex:
- One letter of Pope Gregory VII (r. 1073-85) begins with the address that: "Gregorius, servant of servants of the God [the Pope], sends to King Demetrius (rex Demetrio) [Kniaz Isjaslav of Kiev (d. 1078)] and Queen, a salute and apostolic benediction......(Reg. Greg. II-74, dated to Apr. 17, 1075)"
- History of Norway (Histora Norwegie), Norwegian Latin historical writing customarily dated to the 3rd quarter of the 12th century, narrates how later King Olav Tryggvason of Norway (d. 999/1000) was ransomed and grew up in Russia, with reference to the king/ duke of Novgorod, in its chapter 17: "He [Olav Tryggvason] was ransomed from there [Estonia] by one of his kinsmen, also named Olav, who, as luck would have it, had been sent as ambassador by the Russian king (a rege Ruscie< by rex Ruscie) in order to gather taxes there, so that the boy Olav spent some years in hiding with him in Russia. When he was about twelve years old he courageously avenged his upbringer in the middle of Holmgard market-place. This unheard-of act of by a boy of scarcely a eleven years promptly came to the ears of the king [or duke of Novgorod] (auribus regiis), with the result that Olav was presented to him and afterwards adopted as his son [Ekrem, Mortensen & Fisher trans. 2005: 90f.]"
Mainly based on this contemporary usage, a few recent researchers on medieval Russian history propose that it should be translated as a king for the ruler of the Rus' from the 10th to the early 13th century (Cf. Rappensperger 2017).
References:
- Ekrem, Inger & Lars Boje Mortensen (eds.). Historia Norwegie, trans. peter Fischer. Copenhagen: Museum Tusculanum Pr., 2005.
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- Raffensperger, Christian. The Kingdom of Rus'. Leeds: Arc Humanities, 2017.
(Edited): revises a translation.