Associative entities are used when you need a relationship to be involved in a relationship.
For example:
For a normal many-to-many relationship between Student and Course, we would use just a diamond. However, if we want to associate Enrollment with Teacher, we can turn Enrollment into an associative entity.
Phyiscally, our database looks like this:
Making Enrollment a ternary relationship in which the Teacher is an optional role would have much the same meaning (except it would be denormalized, having a nullable role).
An associative entity is used to implement many to many relationship between two or more entities. This entity is composed of primary key of the each to be connected. The composite entity in the Chen model is represented by diamond shape within a rectangle.