Package Containment means that one Package contains the other, or is nested in the other.
Containment means the containing Classifier does not directly expose the contained Classifier. The Containing Classifier exposes its own interface and can call on the contained Classifier on behalf of the client.
So there is no way for the client to mess with the contained Classifier. The containing Classifier owns the contained Classifier. Example: a restaurant where you order food, and the waiter places your order, the chef cooks the dinner, and it is presented to you.
Relates
The Package Containment relationship can be drawn like this:
