The Flow Filter Flange 355-315mm has been designed to fit the OptiClimate 10000 and 15000 air conditioning units, designed to make the hose fit to the filter. Each flange has screw holes so you can attach the flange to the appropriate surface and as hose connection that is tapered, making it easy to install. You can get flanges in different diameters but this one is specifically made for the 10000 and 15000 units. The numbers 355mm and 315mm relate to the diameters of the filter and hose. The first number, 355mm, shows the diameter of the filter to which the flange can be attached. The second number, 315mm, is the flange connection diameter that allows the correct hose to be connected. It's always worth checking the required diameters for your unit to you make sure you get the correct flange size.