The Coefficient of Friction establishes how much effort it takes to move an object across the face of the tile dry or wet. This is vitally important when choosing floor tile in an attempt to minimize slip and fall injuries. For example, the American with Disabilities Act (ADA) requires that floor surfaces be stable, firm, and slip resistant. The ADA recommends a COF of 0.6 or greater on flat surfaces and 0.8 on ramps and inclines, but there are no laws or standards specifying the COF.
Furthermore, the method of measurement must be specified in order for this to have technical meaning. In the US, we use the ASTM 1028 sled method where a 50-pound weight is placed on a sled with a special material (to represent a shoe sole) contacting the tile which is then pulled by an operator using a fish-scale. It is a dry measurement although it can be performed wet with water for investigation. The pull force needed to just start the sled moving, divided by the 50-pound weight determines the COF. Example: if it takes 30 pounds of pull to start the sled, divide this by 50 and you get 0.6, just enough for the ADA recommendation.