Specifies the grid spacing for rendering a calibration pattern. The object only becomes a calibration pattern if the Filename node is not specified. Otherwise it will be rendered as the specified model.
The manual only shows the GridSpacing node here, because the presence of this node decides whether the object is rendered as a model or as a pattern. You can also specify all other nodes of a pattern specification and the pattern will get rendered with the correct texture that encodes the specified type of pattern.
The grid spacing of the pattern in mm.