Frame notch for lowered car, so that the axel doesn't make contact with frame of car. Description from a company overseas that does it:
"First jack up the hub to see where the strut would hit the chassis. We roughly mark the center of it, then use a 75mm hole saw to cut it out. We usually come up around 10/15 mm from the bottom of the chassis, cut it out, then level out sides with a grinder. You'll then need to make a piece to fit in its place. We recommend bending some 3mm to 5mm steel to weld on, but we have saw guys use pipe from an exhaust, just cut down. Weld it in, welding as much as you can both back and front of the chassis rail. From there, it's just clean it up, some rust protectant and some paint."