Flashback horrors! I used to have to use that function!
Gah!
[edit] Oh, but to relate that to an adding machine... say every thing you add up is the cost of an item plus 5% tax, for example. Under the sum setting, what ever function you set the adding machine to do (add 5% tax to the input amounts), it will provide you said total sum.
So, instead of having to externally add the 5% to each line item, the adding machine will add that to every sum you put into it.
Properly set, you could type into it... 10+ 6+ 9+ 4= 30.45
Index 
Flat 
Let me break this down for you in layman's terms.
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