Why is there a need to specify the same set expression twice?

[#1]

Why do I have to specify the same set expression ( {<Year={2012}>} ) twice? First time for sum() and second for max().

=Max({<Year={2012}>} aggr (sum( {<Year={2012}>} Sale),Company_Name,Product))

Initially I though there would be enough to state it for sum, so that only the rows for 2012 would be passed on to the max().

I have taken the example from:

http://www.analyticsvidhya.com/blog/2014/02/aggr/

The blogger states that the expression is needed twice, but unfortunately not why.

[#2]

I believe it is only a waste of processing

The Set Expression with the definition only in the sum statement works perfectly!

=Max( aggr (sum( {<Year={2012}>} Sale),Company_Name,Product))

A "set statement" creates a static set of data, should be included only in the part that defines the value

[#3]

As I stated in my initial post and stated by the original blogger, both are needed to get correct result.

[JonnyPoole]

Each aggregation function can leverage a unique set statement whether or not its inclusive of the inner set or not. This is for maximum flexbility

Below for example, the inner set is on AccountID=1 , but the outer set is on AccountID=2

so what happens ?

the outer is what is used on where to render the values, but the inner set  is being used for the subtotalling .

pretty flexible.