Standard diaphragm valves, or weir valves, seal the diaphragm to the valve body by sandwiching it in-between the body and the actuator.  When the valve is actuated is closes on a weir which blocks flow between the two ports.  For proper drainability the valve must be installed at a specific drain angle.

Radial diaphragm valves have a clean static seal around the circumference of the diaphragm.  There is no need for a weir to control flow as the seat of the diaphragm seals across the port of the valve.