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