By default, the FormatValue is empty. The FormatValue property allows you to define an expression that will be applied to each individual value before it is passed into the aggregate function for processing. This property is particularly useful when you need to manipulate or transform the values in a certain way before they are aggregated. On the other hand, the Base property defines the core aggregate function, with "sum" being the most commonly used function in typical scenarios, as explained below.

If your goal is to adjust or convert the values before they are aggregated, the FormatValue property is the appropriate choice. However, if you need to perform any changes, formatting, or adjustments after the aggregation has been completed (such as modifying the final result), you should use the FormatResult property. This allows for more flexibility in how the data is presented or manipulated at different stages of the aggregation process.

If the FormatValue 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 FormatValue expression is correctly defined and valid, allowing the aggregate function to work as expected.

For instance:

• you want to get the total for negative values or to count the positive value only. In this case, you can add a new Aggregate object such as Aggregates.Add("negative", "sum").FormatValue = "value < 0 ? value : 0", and so the negative Aggregate function gets the total of negative values only. 
• The  Aggregates.Add("positive", "sum").FormatValue = "value < 0 ? 0 : 1", counts the number of positive values. 

The expression of the FormatValue property supports the following keywords:

• value, indicates the value being evaluate
• vcount, specifies the number of elements in the set, so far

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