Emperor
The first Austrian Emperor was the last Holy Roman Emperor Francis II. In the face of aggressions by Napoleon, Francis feared for the future of the Holy Roman Empire. He wished to maintain his and his family's Imperial status in the event that the Holy Roman Empire should be dissolved, as it indeed was in 1806 when an Austrian-led army suffered a humiliating defeat at the Battle of Austerlitz. After which, the victorious Napoleon proceeded to dismantle the old "Reich" by severing a good portion from the empire and turning it into a separate Confederation of the Rhine. With the size of his imperial realm significantly reduced, Francis II, "Holy Roman Emperor" became Francis I, "Emperor of Austria". The new imperial title may have sounded less prestigious than the old one, but Francis' dynasty continued to rule from Austria and a Habsburg monarch was still an emperor ("Kaiser"), and not just merely a king ("König"), in name.
King of the Romans
Originally all noblemen present could vote by unanimous acclamation, but later a franchise was granted to only the most eminent bishops and noblemen, and according to the Golden Bull of 1356 issued by Emperor Charles IV only the seven Prince-electors had the right to participate in a majority voting as determined by the 1338 Declaration of Rhense. They were the Prince-Archbishops of Mainz, Trier and Cologne as well as the King of Bohemia, the Count Palatine of the Rhine, the Saxon duke, and the Margrave of Brandenburg. After the Investiture Controversy, Charles intended to strengthen the legal status of the "Rex Romanorum" beyond Papal approbation. Consequently, among his successors only Sigismund and Frederick III were still crowned Emperors in Rome and in 1530 Charles V was the last king to receive the Imperial Crown at the hands of the Pope (in Bologna). The Golden Bull remained effective as constitutional law until the Empire's dissolution in 1806.