It allows groups to be added. If you have a group called Income with various accounts then you have a group called Cost of Sales with various accounts you can then total those two groups with a heading. But you have to use the codes to get it to function correctly - Income code 1, Cost of Sales code 2 Gross Profit code 3. (remember that any expenses groups need the tick box clicked)
If you have a look near the bottom of topic What account does corporation tax go under? you can see it being used to cater for/separate corporation tax