They are allowed as long as they don’t contain synthetic fertilizers. These are products where fibers (peat or coir in the case of most grow plugs, and mineral fibers in the case of rock wool) are compressed and held together with binding materials. The structure provides a porous growing medium that provides structural support to plant roots while allowing the right mix of air and water to reach the roots.  Certified Naturally Grown views these products as essentially serving the same function that plastic pots serve on soil-based farms.