These are the numbers that matter for the Dynamic chair. The N stands for Newton and corresponds with the resistance of the mechanism that causes the seat to tilt together with your weight shift. Heavier people need heavier mechanisms. Our lightest mechanism is 350N, our heaviest - 650N, but most common are 450N and 550N.
We now supply the sleeve (gas cylinder cover) when specifically requested. If you make this known to us at the time of placing your order, we will include it to your order at no extra charge. If you would like to specially order the sleeve only the the shipping cost will be charged.