BeSafe Stretch lower tether straps are needed to properly install the seat in your car. They are attached to the floor of your car, so that you can attached the retracting lower tethers to these anchorage straps.
One pair of lower tethers are included with the seat. If you have several cars it might be convenient with an extra pair of Stretch lower tether straps so you don’t have to de-install when moving the seat between cars. Buy an extra pair if needed.