The Papal Bull Aeterni regis was issued on 21 June 1481 by Pope Sixtus IV, simply confirmed the substance of the Treaty of Alcáçovas, thereby reiterating that treaty's confirmation of Castile in its possession of the Canary Islands and its granting of all further acquisitions made by Christian powers in Africa (and eastward to the Indies) to Portugal.
The Bull is the legal basis of Don Christopher Columbus's authority as viceregal agent of the Spanish crown in the Americas and one of a series implementing the intent of Romanus Pontifex which was rescinded/redacted by Sublimus Dei.