True if the style is a built-in style. Read-only Boolean.
Example
This example creates a list on worksheet one that contains the names and built-in status of all the styles in the active workbook.
r = 0
Worksheets(1).Activate
For Each s In ActiveWorkbook.Styles
r = r + 1
Cells(r, 1).Value = s.Name
Cells(r, 2).Value = s.BuiltIn
Next