The Optionality section lets you specify whether a relationship
is optional or mandatory. For example, you could require that all
Document objects have a related Writer object but not require that
all Document objects have a related Illustrator object. When you
attempt to save an object that has a mandatory relationship that
is not set (so the relationship is null),
Enterprise Objects refuses the save and displays an error message
stating that the object being saved has a mandatory relationship
that must be set.