I did puzzle at the markings....Carling is labeling the upper stud as line in but MidNite epanels all use the lower stud for line in....it is tradition in the industry that the top is line in for most industrial applications but it's not a hard rule. It trust the crew at MidNite to do it right. I did download full line docs from Carling on the lines that I use.....A,B,C,E & F, And all are not polarity sensitive...according to Carling..the A line is bat handle toggles,DC and the B line is much like the C line (MNDC) but lower voltage....marine. Really good product line by a good company. Used Carling for a long time.