AcSmSheetSet.SetDefAltPageSetup
From AutoCAD Sheet Set Manager API

Sets the IAcSmNamedAcDbObjectReference component that references the default page setup within the template containing the alternative page setups.
Syntax
SetDefAltPageSetup(
pAltPageSetup As IAcSmNamedAcDbObjectReference) As Object
| pAltPageSetup | Input object that references the default page setup within the template containing the alternative page setups |
| Comments? |