Ceramic tile is tested, graded and rated for several criteria, the following 3 ratings are generally considered the most important:

PEI Rating:

Although there are no industry standards, most tile is rated for use by the Porcelain Enamel Institute (PEI) abrasion test recommended by the American Society Testing Materials (ASTM)

Class 1 (PEI 1)
Is generally recommended for interior commercial and residential walls only.
Class 2 (PEI 2)
Is generally recommended for interior commercial and residential walls and residential bathroom floors.
Class 3 (PEI 3)
Is generally recommended for interior commercial and residential walls, countertops and residential bathroom floors.
Class 4 (PEI 4)
Is generally recommended for moderate to heavy traffic, including all residential and most commercial applications.
Class 4+
Is generally recommended for extra heavy traffic in all residential and commercial applications.

Water Absorption:

ASTM C-373 rates the amount of water absorption of bisque. 7-20% Non-vitreous, 3-7% Semi- vitreous, 0.5-3% Vitreous, less than .5% Porcelain Impervious. Porcelain impervious tile is safe to use in high moisture applications such as steam rooms & swimming pools.

Glaze Hardness:

Moh's Scales lists 10 minerals according to their hardness (from talc at 1 to diamonds at 10) and the minerals ability in the scale to scratch the surface. Most glazes used on ceramic tile fall in the five to six range, which is also slightly harder than most steels. Case-hardened steel, such as what is used in drill bits used for drilling holes in steel, is approximately six and will scratch most glazes. Some glazes used on ceramic tiles designed for floor use cannot be scratched by a case- hardened drill bit. Quartz, number 7 on the Moh's scale, will scratch all but the hardest unglazed ceramic tiles.