How to find view only relationships ?

Sure, A good example, is the native datamodel for Part, Part Document, Part BOM and Part instances Part is an Itemtype :

• It has a "Classic" Relationships to the Document itemtype. the relationship is called "Part Document". A document can be defined independently from the Part.
• It has an "Almost Classic" Relationship to the Part (itself) itemtype. The relationship is called "Part BOM". Parts can be created independently and then related in the Recursive BOM.

If you look at the "Part BOM" Itemtype (yes relationships also are itemtypes !) it has a null relationship called "Part Instance". A part instance cannot be independant from a Part BOM it is a more precise granularity of the BOM relationship and it tipically contains postionning of the component in a parent assembly context. Other examples would be a purchase order containing order line items. Each line is stritcly dependant of the order. If you delete the order you delete the order-lines, so this would be achieved with a null-relationship. The more you use Aras Innovator, the more you will find examples in the datamodel and the more you will understand why you would need to select one of the three relationship types that I mentionned when you will configure the solution for your own projects.