You can enter transformation port names in an expression.
For connected transformations, if you use port names in an expression, the Developer tool updates that expression when you change port names in the transformation. For example, you write an expression that determines the difference between two dates, Date_Promised and Date_Delivered. If you change the Date_Promised port name to Due_Date, the Developer tool changes the Date_Promised port name to Due_Date in the expression.
You can propagate the name Due_Date to other non-reusable transformations that depend on this port in the mapping.