certifies brake power, as braking is very important from safety point of view.
for coaching rake, the braking power should be 100% of the rated power (i.e. when brakes are applied it should stop within specified limits, no exceeding) at starting station and 90 at intermediate. if there is any issue with any coach like brake binding, leakages in BP, issues with compressor etc these can compromise the braking system's efficacy and thereby the safety of passengers.