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+).

 PEI 1 is wall only,
 PEI 2 is for walls and residential bathroom floors only,
 PEI 3 is residential floors 
 PEI 4 and above is rated for commercial floors. 
 
Since it is vitreous, it has a higher breaking strenght and is less absorbant (below 7%) than clay tile and is normally rated for interior or exterior use.

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).

 Standard grade tile has met all the size, finish, color and dimension requirements and will have defective rating of 3% or less.

 Seconds grade tile are tile that have been culled out of standard grade runs due to variations in size, finish, color or dimensions.

 Builders grade tile is uninspected and will usually have more than the standard 3% defects.