One reason you might want multiple Data-Relations between the same buffers is because it is efficient to fill the ProDataSet using one parent-child hierarchy, but to navigate using a reverse hierarchy. A data-rel-spec such as "FOR Customer, Order" might be good for navigation, but if the source is based on order.orderdate, then "FOR Order, Customer" is better for filling the ProDataSet.
For dynamic ProDataSets, an optional parameter of the
ADD-RELATION method is used to define a Data-Relation as inactive. For more information about this method, see
CreatingData-Relation objects.