The phrase "friction coefficients" refers to a measure of how much resistance or "stickiness" there is between two surfaces when they slide or rub against each other. Full definition
This allows the system to prevent a wheel from spinning in extreme conditions on slippery road surfaces or when the two rear wheels are experiencing significant differences in friction coefficient. (press.bmwgroup.com)
The rings are reaching their maximum friction coefficient at that time do to maximum distance traveled within the bore. (mechanics.stackexchange.com)
The resulting apparent friction coefficient of 0.08 is considerably smaller than static values for most rocks. (science.sciencemag.org)