Wall tiles have a porous body with a water absorption above 10% (group BIII in EN 14411). The porosity keeps the tile light and dimensionally stable during firing, and makes it easy to cut and drill on site.
Because the body is porous, the glaze does the work: it carries the colour and makes the face washable and impervious. Wall tiles are meant for interior walls. They are not designed for floors, for frost or for heavy mechanical loads.