King of the Romans=King of Germany (Rex Teutonicum). Elected on East Frankish soil (usually at Frankfurt), then crowned generally at Aachen or Frankfurt.
It was originally a union of Franconia, Saxony, Swabia, & Bavaria, plus the Saxon marches in the northeast (Brandenburg, Mecklenburg, Meissen, Lausitz, etc.) & the Bavarian marches in the south east (Austria, Styria, Carinthia, Carniola). Lotharingia was added in 925 by Henry I the Fowler, including all the Low Countries except Flanders & Artois.
Italy & Burgundy (Franche Comte, Bern, Savoy, Provence, Dauphine, etc.) were not parts of the Kingdom; they were parts of the Empire. The King of the "Romans" did not rule either unless he was also Emperor.