Garden TAP TO BACKFLOW prevention device, to Pressure Pipe
Your water connection from your Tap will comprise of 1 of each of the Bush, Dual Check Valve, and the End Connector.
The Y piece enables a 2nd Tap to be installed so that you still have a Tap for Garden Hose use, providing your existing Tap has a 15mm Male Thread on the inlet side. Anything else will require additional Plumbing fittings.
If you have an alternative water supply (i.e. not Town Water), then the Dual Check Valve is not a requirement.