The backing on sandpaper can be made from various materials and come in different thicknesses:
Backing Material: Most sandpaper has a paper or cloth backing. Paper backing is less expensive while cloth backing is more durable.
Backing Grade: Backing materials are graded by weight. Paper backings are given alphabetical letter grades from A (thin) through F (thick). Cloth backings are graded J (thin) or X (thick). The grade of the backing material is printed on the back of the sandpaper. In most cases, the manufacturer chooses a grade of backing that corresponds with the grit, so it’ll hold up (or be flexible) for the job, with coarser grits on heavy backing and fine grits on thin backing. However, you may find that economy packs of paper are cheaper because they have a lighter weight backing.