Returns the base item index of a data field in the PivotTable.

 public int BaseItemIndex{ get;}

Public Property BaseItemIndex() As Integer

Calculated fields return -1. You can check if a PivotField is calculated 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.