The Porcelain Enamel Institute (PEI) has a scale that was created to determine the resistance of an enameled surface in relation to the traffic and wear the surface will endure. Once assigned a rating, tiles can be evaluated to better determine which tile to use for a given application.
The PEI Rating of ceramic tile is one factor to help determine it's appropriateness in a project
PEI 1 : No foot traffic
           Wall use only in residential and commercial applications. This type of tile should never be used underfoot.
PEI 2 : Light Traffic
           Both wall use and flooring areas that receive little traffic, such as residential bathrooms.
PEI 3 : Light to moderate traffic
           Countertops, walls and floors that receive normal foot traffic are best suited for PEI-3 rated tiles. This is a good,
           general purpose tile for all residential (but not commercial) uses.
PEI 4 : Moderate to heavy traffic
           All residential applications, as well as medium commercial and light institutional, work with PEI-5 rated tile.
PEI 5 : Heavy to extra heavy traffic
           All residential and heavy commercial and institutional foot traffic. Typically this is used only for flooring and is
           rarely attractive enough for interior residential applications.
V1 - Uniform appearance where differences from tile to tile are minimal.
V2 - Slight variation where there may be differences in texture and/or shade within similar colours.
V3 - Moderate variation where the colour on one tile will be inducative of colours in other tiles;
        however, the amount of colour on each may vary.
V4 - Random variation where multiple colours and shade produce a unique blend of tiles.
A good quality glaze on ceramic tile is non-porous at time of manufacturing. 
Therefore water absorption pertains only to the tile's body. The water absorption rate is important 
information in selecting the right product for outdoor use. If the tile absorbs more than 0.5% moisture, 
it is not suitable for outdoor use in colder climates where tile may freeze and crack.
The skid resistance scale is used to determine the degree of slippage on a tiled surface. 
While assessing different tile surfaces, wet and dry conditions along with the speed of the subject are 
monitored. Also, the required force to move that subject and the angle of the tiled place are considered.
The skid resistance scale is used to determine the degree of slippage on a tiled surface. 
Slip Resistant  -   Coefficient of friction is 0.60 or greater (wet). 
Conditionally Slip Resistant  -  Coefficient of friction is 0.50 to 0.59 (wet).
Questionable  -  Coefficient of friction less than 0.50