The PivotSummaryDisplayType.Index summary display mode displays index values that reflect the summary values' importance in their row and column context. These values are calculated as follows:
((value in cell) x (Grand Total of Grand Totals)) / ((Grand Row Total) x (Grand Column Total))
In the following image, the SummaryDisplayType properties of the two data fields are set to Default and Index respectively.
For instance, the "Beverages - UK - Index" cell value is calculated as follows: 0.911 = (68,057.1 x 775,779.61) / (216,298.97 x 267,868.16), where 68,057.1 is the actual sum calculated against the Product Sales field for the "Beverages - UK" cell; 775,779.61 is the Grand Total value calculated against all cells; 216,298.97 is the Grand Total value calculated for the "UK" column; and 267,868.16 is the Grand Total value calculated for the "Beverages" row.