According to aggregation, the relation between any two entities is considered as a single entity. Any relationship with its respective entities is aggregated in a higher level entity.
For instance, Any course entity is offered by the center entity as a single entity in the corresponding relationship with any other entity visitor. According to this, if a person visits any coaching center, then the person may never enquire about the course or the center alone. Instead, the person would inquire about the course and the center both. He is supposed to ask both of the queries to take further steps.