I think that 'box feed' refers to the way the feed dogs move.
Regular feed dogs in a gentle arc resulting in fabric moving unevenly at the start and end of each stitch. /--\_/--\_/… like a sine wave.
Box feed is different in that the feed dogs move straight up, forwards, straight down. |--|_|--|_|… more like a square wave. This is supposed to move the fabric with greater evenness.