how did they designated heirs?
The Emperor could, and often did, have a relative (usually a son) elected to succeed him after his death.
King of the Romans
The Holy Roman Empire was an elective monarchy. No person had a legal right to the succession simply because he was related to the current Emperor. However, the Emperor could, and often did, have a relative (usually a son) elected to succeed him after his death. 
which of the following was a significant factor in both the german wars for unification UNANSWERABLE what were the first efforts at this unification? Programs like the Wartburg rally and the Hambach Festival were organized why was a unified state founded? To replace the German Confederation, a loose association of sovereign states, and the highly decentralized Holy Roman Empire. when was the empire that you just mentioned formed? UNANSWERABLE what political structure changes happened during this empire's period? During the 13th century, a general structural change in how land was administered prepared the shift of political power towards the rising bourgeoisie at the expense of the aristocratic feudalism can you mention some of the roman kings? Charles IV and Charles V what were some of the later developments of the roman kings? The rulers of the Empire thereafter called themselves "Emperors" without going to Rome or soliciting Papal approval, taking the title as soon as they were crowned in Germany or upon the death of a sitting Emperor if they were elected as heir to the throne.
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King of the Romans [SEP] Heirs designate
The Holy Roman Empire was an elective monarchy. No person had a legal right to the succession simply because he was related to the current Emperor. However, the Emperor could, and often did, have a relative (usually a son) elected to succeed him after his death. This elected heir apparent bore the title "King of the Romans". The election was in the same form as that of the senior ruler, and theoretically meant that both men were equal co-rulers of the Empire. In practice, however, the actual administration of the Empire was always managed by the Emperor, with at most certain duties delegated to the heir.