Can someone please clear up what is meant by Primary and Secondary underlays?

[ckramer]

When creating a fill with an underlay and you list edgewalk as the "primary underlay" with fill as the" secondary underlay" does it still always sew the fill underlay first and then the edgewalk?? I realize I can manually create the edgewalk first then add a fill but why does the DS program do this....is this normal...or have I totally misunderstood the idea of primary and secondary underlays? Perhaps it has no reference as to the order of sew??