What is the main difference between the Holy Roman Empire and the Empire of Charlemagne?

The Frankish Empire reached its heyday under Charlemagne or Charlemagne, but in 843 it was divided into 3 parts by his sons. The Holy Roman Empire is the heir to the East Frankish Kingdom with part of the Middle Kingdom. A distinctive feature is that the empire of the Franks united all the peoples of Western and Central Europe, and the empire was the main base for the formation of the German ethnos. The Frankish empire can be called the cradle of the nascent nations of Western Europe, which will already arise later, in national kingdoms and supranational formations – empires.