The McAlpine BT4 is a type of plumbing fixture called a "P-trap," which is used to prevent foul odors and sewer gases from entering a building through the drain system. The "P" shape of the trap holds a small amount of water, which creates a barrier that prevents gases from passing through. The BT4 model has a water seal depth of 50mm, meaning that the depth of the water seal is 50mm deep. This is the distance between the water level and the lowest point of the trap's bend. The 1¼" multifit outlet refers to the size of the pipe that the trap can be connected to. The "swivel" feature of this trap means that the trap can be rotated or adjusted to fit the plumbing configuration of the installation site. This allows for greater flexibility in installation and ensures a tight, secure fit. Overall, the McAlpine BT4 50mm Water Seal Tubular Swivel 'P' Trap with 1¼" Multifit Outlet is a high-quality plumbing fixture that is designed to provide reliable performance and prevent unpleasant odors from entering a building through the drain system. ・ 50mm Water Seal ・ 1¼" Multifit Compression Outlet suits all plastic, copper and lead pipe ・ Conforms to BS EN 274-1:2002