The Back Order Viewer shows you a live list of the back orders currently on a list. To open this window, go to the General tab of a list's properties, and press View Back Orders...
Columns
For each back order, the table shows the puller, the query, the number of requested entries, the number of required entries, and the number fulfilled so far. Usually objects pull one at a time, so requested, required, and fulfilled will often be 1, 1, and 0 respectively.
