Depth:

Enables you to set the distance from the view plane (positive outward, negative inward; the view plane is the plane of the monitor screen) in a direction orthogonal to it, when the Mouse Input is set to On view.

a Click point

b Screen plane

When you specify points by clicking with the mouse (without snapping to special points), the point you specify is assumed to be parallel to the current view plane at the specified depth. Since the screen plane normal is considered to be directed outward from the screen:

A positive depth value implies that points indicated using the mouse lie on a plane parallel to the screen plane, on the same side of the screen as the user
A negative depth value implies that the points entered using the mouse lie on a plane parallel to the screen plane on the opposite side of the screen from the user.

The default depth on all views is 0. In that case, all input from the mouse is on the screen plane in each view.

Note: Remember that unless you are viewing the model from a position perpendicular to the Work Plane, the depth doesn't coincide with the z-coordinate of a point provided using the mouse. The z-coordinate and depth coincide only when the z-axis is orthogonal to the screen plane.