By default, the FormatResult property is empty. The FormatResult property is used to convert or format the final result of the aggregate functions. If you need to modify the value before it is aggregated, you should use the FormatValue property instead. If the goal is to apply changes or formatting to the aggregated result itself, then the FormatResult property should be used to directly handle the formatting or conversion.

If the FormatResult expression is not empty and is invalid, the aggregate function may appear disabled in the control's context menu. This happens because the control detects an error or issue with the expression, preventing the proper application of the aggregate function. To resolve this, ensure that the FormatResult expression is correctly defined and valid, allowing the aggregate function to work as expected.

For instance: 

• you want to display the result multiplied by 1.19 to show the VAT, so all you need is to add a new aggregate function like Aggregates.Add("vat", "sum").FormatResult = "value * 1.19", so the result is multiplied by 1.19. 
• Aggregates.Add("colors", "sum").FormatResult = "(value < 500 ? '<fgcolor=FF0000>' : '') + value", shows the values under 500 in red. 

The expression of the FormatResult property supports the following keywords:

• value, indicates the value/result to be formatted
• vcount, specifies the number of elements in the set

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