A SlidingDoor slides as if on a track or railing. A SlidingDoor has two door elements: one remains stationary while the other moves.
The topic for each kind of door describes its unique controls and behavior. Most door parameters are common to all kinds of doors,see Doors
The two elements are offset by the width of each door element. The Depth parameter of a SlidingDoor is actually half the depth of the extents of the SlidingDoor object.
In the door's Parameters rollout, the three check box controls below the Frame area are specific to SlidingDoors.
Flip Front Back: Check this to change which element is in front, compared to the default.
Flip Side: Check this to change the current sliding element to the stationary element and the stationary elment to the sliding element.
For an explanation of how to create a door see Doors