The Flat Plane Splines are intended primarily for use with the various cuttings in the Landscape Kit, either to create a simple flat top or else to follow the contours of the top of a cutting and thus bridge the gap between it and the baseboard. Their spline unit width matches those of the cuttings, but they come in a choice of lengths so that you can choose the one most convenient to fill the gap between the spline and the board. The texture is designed to tie in with all the other Kit assets sharing the MGGI suffix.
You will find further details and examples of their use in my 'Modular Landscape Kit' thread in the Freeware Anouncements section of the Trainz forum.