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For question 1
The following topics in Oracle Policy Modeling User's Guide are helpful
"Understand containment relationships and entity completion" where the concept of an entity needing to be complete is determined via the use of containment relationships.
"Reason across multiple entities" where the containment relationship needs to be known to determine the scope of an entity.
"Use multiple properties files on a multi-developer project" where the containtainment relationship used to connect separate property files.
Version 10.4 of OPM automatically creates a default containment relationship.
For question 5 I believe your question is in regards to topic "Define a data model"; your link brings us to the home page.
For (3) - no, every entity must be contained by another entity - you can only create an entity instance if you have the containing entity instance. This is what I call the "brain-attic" axiom - you shouldn't store knowledge if it is unrelated to the scenario being modelled.
For (5) - you can create a containment relationship between "the child" and "the friend", and that is what the example is illustrating - only "the child" has the containment relationship from the global entity.
(For the meaning of "brain-attic", see http://www.literature.org/authors/doyle-arthur-conan/study-in-scarlet/part-01/chapter-02.html)
Thanks Brad.. However for the point 5.. the OPM helps says..
"Containment relationships are also required for the Friend and School entities, since the above relationships for these entities are not suitable "to-one" relationships. We would define:
Global entity and Friend (the friends): this is a one-to-many containment relationship.
Global entity and School (the schools): this is a one-to-many containment relationship. "
As you point out, if there is always a strict -to-one relationship, you can model it using a straightforward containment relationship.
I.e. if a child can always ever go to one school, then you can contain children in schools.
However, if there is ever a situation where the relationship between children and schools is more nuanced - e.g. they can move between schools over time, or can attend more than one school - and you don't want to duplicate the child into each school they are in - then you will need to have both the children and the schools be contained by the Global entity, and build a relationship, or set of relationships, between them to reflect the actual situation.
Davin has already answered this, so I'll just point out that the example is merely one way to implement a set of containment relationships, and it is up to the rule modeller to decide what the most appropriate source and target entities are for each containment relationship. The global entity is usually chosen as the source entity of a containment relationship because this allows other entities to define many-to-many relationships involving the entity.