1. Use interface type.
2. Ordered type collection has index sub-element.
3. Key element indicates the foreign key referred to entity own the collection.
4. "element" or "composite element" element is used to specify value type contained in collection.
5. "many-to-many" or "one-to-many" elmeent is used to sepcify referred type contained in collection.
6. one-to-many need not an intervening table.(foreign key)