Parent objects on process models when creating a process model

The moment one creates an assignment from a function to another process model, it automatically copies the parent function to that particular diagram. The question is to disable that behaviour for process models and keep it for FAD diagrams''. We did a research and fond out that for our model types Levels 1 to 3 they were derived by Value-added Chain Diagram. So it is standard to have the assignment object created automatically. The customer suggests when he creates an assignment for the function, then this function to be not copied automatically.