A Stop & Waste Valve is a valve used to shut off water, that has a port that can be opened to drain the outlet side of the valve. A Stop & Waste Valve can be any of the various types of valves, ball valve, globe valve, gate valve, etc. but they are able to waste or, drain the water held in the valve and pipes on the outlet side of the valve through a port on the valve when the valve is closed, and the waste port is opened.
The Stop & Waste Valves port is usually a male threaded stub that extends off the valve, the port has a hole in it connecting to the outlet side of the valve. The waste port is usually covered by a cap that has a piece of rubber in it that seals against the waste port when the cap is tightened.
When installing a Stop & Waste Valve it is important to make sure the waste side of the valve is orientated properly so it can drain the outlet or controlled side of the valve, and the waste port should be on the bottom when the valve is installed horizontally to facilitate proper draining.