Kbg, Damn I have seen that same thing and wondered, but I am guessing it means it has a max of total cfm that it can safely even have inline like from the furnace or source connected to it...not sure tho and I have the munches... I bought a Ecoplus Premium Inline fan with a 440cfm at max, and then I got the speed control for it and I am glad I did it is damn loud and probably more then I needed, kinda rethinking it before using it.. Doesn't hurt to do recon does it...:eat: