The relationship between two objects is termed as Association.
In particular when one object owns another object, then that association is called as Composition, but when one object uses another object then that association is called as Aggregation.
Sales Order Header and Items are associated. So when SO header is deleted then SO items are also deleted. So it’s a composition.
Similarly SO Header and Partners are associated. But when a SO header is deleted then the partners are not deleted. So it is an aggregation.
A CDS composition is a specialized/more strict form of CDS association which defines the CDS entity as the parent entity of the composition target. The composition target entity is the child entity and it must define a TO-PARENT association to its parent.
It is not required to maintain ON condition for the composition. The ON condition is derived automatically by the composition child TO_PARENT ON condition.
The example CDSs are-