Isolating or Shut Off Valves are those which are devised to interrupt the flow in a System by means of a mechanically guided closure element. Linear Isolating valves are featured by the movement of the stem guiding the closure element downwards to get the valve closed and upwards to get it open.
The closing and opening actions are featured by a slow movement and usually governed by a multi turn hand wheel. They mean the right choice on compressible fluids such as steam to avoid those quick turning movements that might damage the valve and / or the System because of hydraulic phenomena.