The "Summary by all Accounts" window is similar to the "Open Positions" window, except that all instrument positions are summarized. 

For example, if you have three separate open buy positions of one lot each in USDJPY, the "Summary by all Accounts" window will just show one position of 3 lots, with the average open price in the "AvOpen" column.  If you have three separate open buy positions of one lot each in USDJPY, and one short position of one lot in USDJPY, the "Summary by all Accounts" window will show the buy position as one position of 3 lots, and the sell position as one position of one lot, and the NH ("not hedged") column will show a "2", indicating that you have a net buy exposure of two lots.

This window is also useful when trading multiple accounts.  It will summarize whichever account is selected in the "Account Information" window.  In addition, if you right-click on the gray heading and select "Summary by all Accounts", it will summarize the positions of all accounts managed with your login as if they were a single account.