The U plunge bra is a speciality bra that is designed for really low cut necklines. There are the lowest you can get in the front and still be wearing a bra. If you need it to be lower you will have to go braless (see what bra to wear with a plunge neck for more options).
U plunge bras usually have underwires and side boning to add support. Since you are probably wearing something a little risque if you are going with this sort of bra, fashion tape can add an extra level of protection from a wardrobe malfunction.