U-cups are popular because they only require a small gland space. Usually they are specified in low pressure, low speed environments for both hydraulic and pneumatic cylinders. The seal is achieved by the flared, flexible lips coming in contact with the rod or cylinder wall. Clearance is provided at the heel, resulting in minimal friction. The seal effectiveness increases as internal pressure increases against the inner surface of the lips.
Since the shape is symmetrical this type of piston seal could also be used as a rod seal if necessary.