Key light, Fill light, Hair light, background light. What i understand is that studio flash units are fitted with a continous lamp or modeling light to make your lighting easy to set-up. Now, where are you going to fire the flash? is it in the key light?, hair light?, etc. When you setup 3 studio lights on a particular subject, ie, fill light,key light and background light, are you going to fire 3 flashes attached on those lights?