5 - 6 mm pull cord / intervention rope
Pull cord: this is a lighter rope that you use to pull down a descent rope. This way you can reduce the total weight of rope that you take with you. Otherwise you will need twice the length of the descent rope on the longest descent. With a pull cord you can reduce this to 1 time the length of the down rope and 1 time the length of the pull cord.
Intervention rope means that it is a rope that can be used by the guide / canyoneer / caver to hang out quickly and get to a student / canyoneer / caver / victim to help - free it on the spot. An intervention rope is thinner and more compact to wear it on the belt of the guide / canyoneer and to deploy quickly.