Strong Reference Plane vs Weak Reference Plane

When you load the family into a project and attempt to dimension to it, the strong reference planes with have preference over the weak ones. So if you hover over the element and there is a strong and weak reference plane in close proximity, the tool will snap to the strong one and you'll have to tab to get to the weak one.

If you assign an instance length parameter to reference planes in a family (so you get shape handles in a project environment), the strong and weak ones will behave differently once loaded into a project too. Once loaded into the project, aligning the strong reference plane to an element will stretch the family, whilst aligning the weak reference plane will move the entire family to that element without altering that instance parameter.