This parameter is used to check whether or not the cut is in the XZ (G18) or YZ (G19) plane. By checking the box, Router-CIM will produce the most efficient code possible for geometry outside of the XY plane. This is accomplished by automatically evaluating the tool path face (Coordinate System) for its eligibility to use Arc Interpolation vs. Point to Point. Plane detect will be reflected in your code by either a G18 (XZ plane selection) or G19 (YZ plane selection) if possible, or G17 (XY plane selection) otherwise.
This drawing illustrates the fact that the front and back are both on the XZ plane and the right and left sides are both on the YZ plane.
The G17 plane is the XY plane. If any horizontal cuts do not like exactly on the G18 or G19 faces, then they will be output as G17 code.