This .NET enum wraps AcDbPlotSettings::stdScaleType> ObjectARX enum. 

The stdScaleType property stores the "standard" print scale selection, including scale-to-fit, for the plot settings object. It is not a number like customPrintScale, but is an enum indicating the desired print scale. AutoCAD uses this, along with other information like the plot area, to compute the customPrintScale when useStandardScale is true. While the stdScaleType property always has a value, it may or may not be reflected in the customPrintScale, depending on the value of the useStandardScale property.

Public Enum StdScaleType
  ScaleToFit
  StdScale1To128InchIs1ft
  StdScale1To64InchIs1ft
  StdScale1To32InchIs1ft
  StdScale1To16InchIs1ft
  StdScale3To32InchIs1ft
  StdScale1To8InchIs1ft
  StdScale3To16InchIs1ft
  StdScale1To4InchIs1ft
  StdScale3To8InchIs1ft
  StdScale1To2InchIs1ft
  StdScale3To4InchIs1ft
  StdScale1InchIs1ft
  StdScale3InchIs1ft
  StdScale6InchIs1ft
  StdScale1ftIs1ft
  StdScale1To1
  StdScale1To2
  StdScale1To4
  StdScale1To5
  StdScale1To8
  StdScale1To10
  StdScale1To16
  StdScale1To20
  StdScale1To30
  StdScale1To40
  StdScale1To50
  StdScale1To100
  StdScale2To1
  StdScale4To1
  StdScale8To1
  StdScale10To1
  StdScale100To1
  StdScale1000To1
End Enum

public enum StdScaleType {
  ScaleToFit,
  StdScale1To128InchIs1ft,
  StdScale1To64InchIs1ft,
  StdScale1To32InchIs1ft,
  StdScale1To16InchIs1ft,
  StdScale3To32InchIs1ft,
  StdScale1To8InchIs1ft,
  StdScale3To16InchIs1ft,
  StdScale1To4InchIs1ft,
  StdScale3To8InchIs1ft,
  StdScale1To2InchIs1ft,
  StdScale3To4InchIs1ft,
  StdScale1InchIs1ft,
  StdScale3InchIs1ft,
  StdScale6InchIs1ft,
  StdScale1ftIs1ft,
  StdScale1To1,
  StdScale1To2,
  StdScale1To4,
  StdScale1To5,
  StdScale1To8,
  StdScale1To10,
  StdScale1To16,
  StdScale1To20,
  StdScale1To30,
  StdScale1To40,
  StdScale1To50,
  StdScale1To100,
  StdScale2To1,
  StdScale4To1,
  StdScale8To1,
  StdScale10To1,
  StdScale100To1,
  StdScale1000To1
}