By default, the FormatPivotHeader property is "iaggregate ? (caggregate + ' of ' + caption) : caption", which indicates that the column's default caption is being displayed in no aggregate function is associated with the column, and Caption of the Aggregate object associated with the column, followed by of keyword, and the column's caption, something like Sum of Column 1. Use the FormatPivotHeader property to determine the way the column displays the captions in the control.

The following keywords are supported by the FormatPivotHeader expression:

• aggregate, indicates the Key of the Aggregate object associated with the Column, or empty if the Column belongs to PivotRows collection. 
• iaggregate, indicates the index/position of the Aggregate object associated with the Column, or 0 if no Aggregate function associated. This value indicates the 1-based position of the Aggregate object in the Aggregates collection.
• naggregate, indicates the Name of the Aggregate object associated with the Column, or empty if the Column belongs to PivotRows collection. 
• caggregate, indicates the Caption of the Aggregate object associated with the Column, or empty if the Column belongs to PivotRows collection. 
• ilevel, indicates the index of the level where the Aggregate function is displayed. This is valid only for PivotTotals property, else it is -1. 
• caption,  indicates the Caption property of the Column
• icaption,  indicates the Index property of the Column
• display, specifies whether the caption is shown on the control's pivot bar (0), or in the columns header(1)

This property/method supports predefined constants and operators/functions as described here.