This dialog is shown as a result of importing positions.
All importing positions will be listed. To not update a particular position remove the check mark next to that position by clicking on the check box with your mouse. Click on it again to turn the check mark back on. You can also use the "Select All" and "Select None" buttons to check/un-check all positions.
You can sort the list by any column by clicking on the column header. Click it again to toggle between ascending/descending. You can re-order the columns by dragging/dropping a column header to the new desired location.