c-notch, c-section, same thing, basically putting in an arched support bottom frame section and cutting out half of your frame from over the axle to allow more room between axle and frame. From what I've been hearing you might want one with just the DJM kit as you might bottom out some especially with stock shocks, and you definitely need a notch to pull a leaf and do the 4" blocks together (as i'm going to do)
unless you basically have your own shop, it's no do it yourselfer...depending on whether you get a bolt on or weld on c-notch you may need to weld the piece in and at the very least you will need a plasma cutter to remove the unwanted section of frame.