To answer this friend's question, we must first understand the definition of category. Generally speaking, category refers to the highest concept. When we call an object a category, it usually refers to a certain discipline or field, such as social category, economic category, philosophical category, scientific category, rational category and so on. Generally speaking, the scope of objects covered by the concept of category is the largest.

In short:

Social category refers to social scope.

Historical category refers to historical scope.

Eternity refers to the scope of eternity.