This is a check valve. Application: This valve is designed to control the flow direction of the fluid flow and only allows the flow to pass in one direction and prevents it from returning. Specifications on Milk: 800: It indicates a valve pressure class that refers to its maximum working pressure. 1/4: Shows the nominal size of the milk (1 and 1/4 inches). A105N: The body of the valve determines that the carbon steel is the ATM A105. D48: May be the manufacturer's internal code or ID.