Many elements 'need' other elements in model that they reference.
For example, to add a Pin-Joint you must select two Points. The two Points are then termed Reference-Element to the Pin-Joint. The Pin-Joint cannot be in the model without the two Points.
To see the Reference-Elements of an element:
A read-only dialog-box shows 'References', Element Type, and Element Owner.
Note: you can often see all of the Elements in the Selection-Window. However, in the Selection-Window, how the elements reference each other is not clear.