Unscreened topsoil is a clean topsoil with some rocks and clay chunks in it. More rocks and clumps of clay makes for a better filling soil for large areas. Unscreened topsoil doesn't wash away like other soils like screened topsoil or pro-mix soil because of its heavy consistency and its clay content doesn't allow unscreened soil to turn to mud when it is rained on. Use unscreened topsoil when working in areas that needed to be filled 6 inches or more.