Plates and tiles benefit from the use of tile and plate setters or stackers. Shelfstyle setters allow air to move under the large flat objects so they heat more evenly. Avoid heating large flat objects directly on the cooler shelf. If you are firing decorated tiles or plates, vertical setters economize on space, and sets can be stacked to fit even more. Glazed ware needs to be stilted or dry footed or the melting glaze will stick the ware to the kiln shelf, ruining both.
Stilts also provide space for air to move around all sides of the ware.
Porcelain and stoneware can not be stilted. The stilts embed into the ware during firing. Instead, use high fire kiln wash or silica sand on the shelf. Use prop to prevent sagging of porcelain. It is important to evaluate heat flow in your kiln and to make this a consideration in loading.