For “ORDERING SIZE” we mean the real size of your insect screen; on the contrary, if you select “CLEAR DIMENSION” we will subtract 2 mm both in width and height to your given sizes, in order to make the installation easier. For example, if your window size is 1352mm x 1452mm and you select “ORDERING SIZE” you will get an insect screen having the real size 1352mm x 1452mm, while by choosing “CLEAR DIMENSION” the size of your insect screen will be 1350mm x 1450mm.