REF (reference)

REF is a special construct, which looks like a function taking one parameter. It's return value is the same as its parameter.

So at first sight it does nothing at all!

Imagine you have a LengthControlPoint, and its Length expression is defined as :

CustomProperty.specialLength

If you were to move the control point in the GUI, then the expressions would change to a constant value. So maybe Length would now be:

100 mm

The CustomProperty.specialLength wouldn't be affected when you dragged the control point.

Now, if we changed the Length expression to :

REF( CustomProperty.specialLength )

Its value would be the same as before. However, now when we move the control point, instead of replacing the expression with a new value, it updates the expression being referenced.

So after dragging, Length's formula will be unchanged :

REF( CustomProperty.specialLength )

but CustomProperty.specialLength will have changed.

This works for any expressions that can be changed via the GUI, not only ControlPoints.