This profoto 10m extension cord is to be used between a Prohead and a Profoto flashgenerator. You might use it when shooting with two heads on an asymmetric pack like the Pro 10 or Pro 8a to have more space to move the heads. Another usual way of using an extension cord is together with a boomstand, cause the head might be far away from the pack. You can join up several extensions to get longer cords but its recomanded to use as less as possible due to the loss of energy.