I have just started learning engineering this year and I have to understand the Australian wind standard: AS/NZS 1170.2. However, as I was studying through the standard. I am a bit confused on the section 5 of the standard on the action combination factor (Kc). The standard states that if there are 3 effective surfaces in the building, the Kc = 0.8, 2 effective surfaces have a Kc of 0.9 and 1 effective surface has a Kc of 1.0. However, I don't quite understand on what constitutes as an effective surface. Can someone explain what an effective surface means in engineering? Thank you in advance for your help. I would appreciate it very much.