Your production rate is how much area one painter covers in one work day. The Rates page works yours out from jobs you have already finished, using two things PaintClarity already has: the hours your crew logged, and the Paintable Area on the job.
It divides the hours by your Standard work day to get painter-days, then divides the area by that. A 2,400 sq ft job with 60 hours logged, on an 8 hour day, is 7.5 painter-days, so 320 sq ft per painter per day. The headline figure is the middle value across your last 10 completed jobs, not the average, so one mistyped number cannot drag it.
The page measures. It does not change anything. The rate your quotes are built from is the one you set under PaintClarity, Business, Pricing Rules, or per job category under Job Categories. The Labor Days and Crew Cost Calculator starts from that figure and tells you which one it used. Seeing your measured rate next to the rate you quote from is the whole point: when they disagree, you decide what to do about it, because you know why a month ran the way it did and the arithmetic does not.
You will find it under Tools in the menu, as Rates. It is in the free plan.