The Base property indicates the base aggregate function to be used by the current Aggregate object. Use the Add method to define a new aggregate function by overriding a predefined aggregate function or by using a custom expression. If the Base property points to an unknown value, the Aggregate object shows as disabled in the control's context menu. 

The Base property could be one on the following:

• sum, summation is the operation of adding a sequence of numbers; the result is their sum or total
• min, minimum is the smallest value
• max, maximum is the largest value
• count, counts the number of values in the set
• avg, average is the arithmetic mean, which means sum of all numbers divided by the count.
• unique, counts distinct values in the set.