The shape may vary but the objective is the same: more suspension travel. 
A notch is cut into the frame, then reinforced. This will allow whatever the component below it to travel upwards for suspension compression. This results in the lowest drop when you air out and a better ride when you're rolling. A c-notch is usually bolted into the frame above the rear axle for a couple of inches of travel. This is adequate for most static drops. A step notch, or monster notch, is what you see sticking out of the bed floor of the lower trucks out there. It allows maximum travel so you can get lower. Isn't that what we all want?