An example of an association class is given here:
Composition and Aggregation are also types of Associations, just with more defined semantics to what the association consists of.
Can association classes also be part of a relationship by Composition or Aggregation? That is, in the case where there is a many-to-many relationship between two entities, with some additional attributes on the relationship. Or would it be more correct to model that as two separate one-to-many relationships onto a middle entity that hold the additional attributes?