What are exactly nonvitreous, vitreous and porcelain stoneware ?

The key difference between nonvitreous, vitreous and porcelain stoneware is water absorption. Villeroy & Boch leads the field with its range of tiles:

Nonvitreous is basically used as wall covering. It is manufactured using clays, quartzes and limestone. The advantage of this type of tiles is that they are easy to cut and drill.

Glazed vitreous and porcelain stoneware can be used both as a floor and wall covering. They are manufactured using clays, quartzes and feldspars.

Wear on floor tiling is generally caused by sand and other dirt particles that adhere to the soles of shoes and that act like sandpaper under high pressure when the wearer walks over a surface and performs turning movements. As not all glazes possess the same resistance to surface wear, glazed vitreous and porcelain stoneware tiles are categorised according to five groups of differing resistance, namely wear resistance groups 1 (suitable for minimal stress) to 5 (suitable for high levels of stress), and are labelled accordingly.

Unglazed porcelain stoneware has one of the highest wear resistances of all floor coverings. Even after decades of intensive use, this homogeneous material shows hardly any signs of wear.