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Oracle Application Builder Cloud Service makes it very easy to create a parent/child relationship between two objects - in fact it can create this relationship for you automatically when you drop a new table on the edit or details page of another object.
However, when such a relationship is created the child object "belongs" to the parent object - and is not accessible as a "stand-alone" object.
If you want to have the ability to create pages that directly access the child object on its own, then you should create the relationship between the two objects with a reference field.
The video below shows you how to do this and the difference between the solutions: