I am not sure I understand what the specification of the part instance id is useful for in the equation editor. Here is a simple example:
* Imagine I have part which is a box, this box has 3 dimensions: width, height and thickness.
* Now I create an assembly with 2 instances of this box: box<1> and box<2>
* in the assembly equations I write width@box<1> = 55
=> automatically the 2nd instance of the box resizes as well
So why do I need to specify the instance of the part if SW spreads the modification on all the instances?