Simply stated - Strong references will allow dimensions, temporary or permanent, to snap to them. This applies to the reference planes used in Families. See explanation below directly from the HELP menu. When you place or select a door or window in a view it places temporary dimensions from the center line of the door or window to the closet building element (i.e. cneter line of a wall). That's because the center line for both the door or window and the wall are the strong reference planes defined in the families.
Strong and Weak References
To dimension families placed in a project, you need to define family geometry references in the Family Editor. You set geometry references as either strong references or weak references.
A strong reference has the highest priority for dimensioning and snapping. For example, you create a window family and place it into a project. As you are placing the family, temporary dimensions snap to any strong references in the family. When you select the family in the project, temporary dimensions appear at the strong references. If you place a permanent dimension, the strong references in the window geometry highlight first. A strong reference takes precedence over a wall reference point, such as its centerline.
A weak reference has the lowest priority for dimensioning. When you place the family into the project and dimension to it, you may need to press TAB to select a weak reference, as any strong references highlight first.
NOTE:You may also be able to zoom in on the model to highlight weak references, as elements in the model appear farther apart as you zoom in.
Mel Persin, AIA
AEC Technology Consultant
Technology to Visualize and Realize Solutions
Modeling the Future/Drafting the Past