The "Effective Due" field in a service record identifies an estimated due date of the work. This exists to make it practical to sort records in a way that includes due dates, without all of the records without an explicit due date appearing at the end of list!
The effective due date is of course just the due date, if that has been set. Otherwise, it's an estimate, with default work time of 2 work days, at a 10% work rate.
When specifying this field in a customized sort or display, either of the names "date_due_eff" or "Effective Due" can be used.