I know my question is a little vague and is based on Eurocentric/Marxist education I got. In its context, it makes sense to talk about vague periods of time. Prehistoric times with no complex economic relationships, Slavery times in classical era, Feudal relationships in Medieval times, Capitalism afterward. It makes sense when we talk about Europe: before certain time most of it was either ruled by the economies relying on slavery; later those empires crumbled and for most of the Medieval era Europe and the Middle East condemned slavery. While it was still practice it was no longer a basis for the economy.

What am I missing is why world history in general still is divided into same periods. The medieval era is still used even if people talk about India or China or even American people even if another term is used, like the Post-Classical era. I understand why the Modern era is common for everybody; it's the time of globalization and everyone is touched by this process. But what is the difference between the Classical era and the Medieval era for most of the world?