Tools > Alignment > Depth Plane establishes the position of the “depth plane,” a tool that limits selections to points on one side or the other.
The Depth Plane helps in confining a user’s selection to the primitives that lie on one side. The Depth Plane is activated by the checkbox at Display Manager > Accessories > Depth Plane. When the depth plane is active and you select a region, you will not be able to select primitives on one side of said plane.
To choose a plane as the depth plane:
Click Tools > Alignment > Depth Plane.
Choose one of the standard methods for aligning the plane from the Align Plane drop-down menu.
Click OK to accept the new plane, or click Cancel to exit the command at any time.