Many elements 'need' other elements in model that they reference.
For example, a Pin-Joint 'needs' two Points. It must 'reference' these other two Points.
They are called 'Reference Elements'.
To see the Reference Elements that an element needs:
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, how the elements reference each other is not clear.