Tile has multiple ratings, five for glazed, 4 for unglazed tile. Look for the tile's grade (Grade 1 is highest quality, grade, 2 is similar but less expensive & suitable for floors. Grade 3 not heavy-duty enough to walk on, intended only for walls), PEI rating (wear rating only for glazed tile by Porcelain & Enamel Institute on ability to resist abrasion), water absorption (consideration if tile to be used in wet or outdoor area: Nonvitreous tile absorbs more than 7% of its weight in water and is inappropriate; Semiviteous tile absorbs between 3% and 7% of its weight in water and it too is only appropriate for use in dry, indoor areas; Vitreoustile absorbs only 0.5% to 3% of its weight in water for tile in wet or outdoors; Impervious absorbs less than 0.5% of its weight in water), coefficient of friction (COF refers to a tile's natural resistant to slip and it's measured by the force required to slide an object across a surface divided by the object's weight - lower is less friction and less traction), frost safety (whether can withstand freeze and thaw cycles) and tone (intentional variation from tile to tile to mimic the look of natural stone).