A many-to-many relationship is a relationship between two entities where instances in one entity have zero, one, or more related instances in the other entity. In IDEF1X notation, a many-to-many relationship is drawn as a solid line with a solid dot on both the child end and the parent end. In IE notation, a many-to-many relationship is drawn as a solid line with crows feet on both the child end and the parent end.
If you choose to include many-to-many relationships in a logical model, you may encounter some design problems. To avoid design problems that may be caused by many-to-many relationships, database designers frequently add associative entities to act as links for two ambiguously related entities. If you insert an associative entity between the two entities involved in a many-to-many relationship, you can change the relationship into a series of one-to-many relationships and use the new relationships to clarify how the data model works.
You can automatically resolve many-to-many relationships. When a many-to-many relationship is resolved, an associative entity is displayed in the logical model and also eliminates the many-to-many relationship. You can resolve the relationship when you right-click a many-to-many relationship and click Create Association Entity, or by clicking the Many to Many Transformation icon 
on the Transformations toolbar.
Note: When you select DM (Dimensional Modeling) notation for your physical model, the associative table is a fact table.
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