Porcelain tile is kaolin based and considered vitreous. Most have a fired glass surface which has the same ratings as ceramic tile (PEI 1-4+).
There are some porcelains that have color added to the porcelain so the base will match the general color of the glaze. This is refered to as through body porcelain and typically costs more than the standard white or light gray uncolored bodied tile.
The advantage of the through body porcelain is that chips in the glaze are not as noticeable as they are in ceramic or nonthrough body porcelains.
Grades of Ceramic and Porcelain Tile.
There are three grades of fired tile, standard (STD or Class 1), Seconds (STDII or class 2) and builders grade (unclassed).