Pads fall into two basic categories: bladder control pads and booster pads.
Bladder control pads have a barrier backing that prevents liquids from flowing through. Because of that barrier, they are not recommended for use inside another incontinence products (such as a diaper).
Booster pads increase the absorbency of another incontinence product (such as a diaper). They are designed to fill to capacity and then to allow the excess liquid to flow into the diaper. They do not have a waterproof barrier, so they are not recommended for use in regular underwear.