These are defined according to the widely used measures of slip risk based on biomechanical work to establish the required firction for normal walking, on a level surface, at normal pace. Slopes and increased pace increase risk, so whilst 1 in 1,000,000 may seem incredibly low, it would allow for some riskier activity. Definitions:
More than 1 in 2 people need more friction
Fewer than 1 in 2 people need more friction
Fewer than 1 in 200 people need more friction
Fewer than 1 in 1,000,000 people need more friction
In reality, with oil and grease contamination on a smooth surface, no footwear is likely to reach the low risk category, but this assessment can help to seperate footwear that is more suitable, where perhaps the risk on water is the same.