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Returns the base field used in calculating the values for a data field that are shown in the PivotTable.

 public SourceField BaseField{ get;}

Public Property BaseField() As SourceField

Value fields return null. You can check if a Pivot Table field is a value field with the IsValuesLabel Property

When calling SetShowValueAs(), certain ShowValueAsTypes use a source field as a base field and an index for one of the values in the data source column associated with the source field.