Read this article to better understand the many uses of SUMPRODUCT... http://www.xldynamic.com/source/xld.SUMPRODUCT.html
In short, in this case, you are comparing three named ranges to specific criteria. When you compare this way, you end up with arrays of TRUE and FALSE results. The arrays are then multiplied against each other yielding a final array of 1's and 0's (TRUE*TRUE=1, TRUE*FALSE=0, FALSE*FALSE=0, FALSE*TRUE=0). This array is then multiplied by the respectively positioned values in the sum array (Amount). Then that final result is summed to get a single value (sum of all Amount values where all 3 conditions checked were TRUE in the same rows).
In the above formula, you are doing 2 separate SUMPRODUCT functions, and subtracting the result of one from the other to get a final number.
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