In rectangular coordinate systems, grid planes define the grid line location with respect to the X- or Y-axis of the coordinate system. Grid planes are generally parallel to the X-Z or Y-Z plane of the coordinate system, but can be rotated (sloped) after placement. The grid line is defined by the intersection of the grid plane with the elevation plane. Optionally, you can place grid lines at all or some intersections.
In radial coordinate systems, you create your design coordinate system, then create the elevation planes, and then create the cylinders and radial planes. The intersection of the radial planes and the elevation plane create grid lines.