Welcome Eric! Was the old software ('Pace Plus Evaluator') a product of Sartin (aka Inland Empire, O Henry House, Sartin Methodology Inc depending on the circa) OR was it some other parties' knock off 'in the style of Sartin'.
If the latter, they could refer to anything! They could be taking Sartin's early assertion that PACE = CLASS = SPEED and 'branding' some kind of pace+whatnot as 'pace class' a la Sartin.
If it was an early Sartin product, it could simply be some kind of pace-call (2nd call) measurement of velocity turned to a rank or 'magical number' equivalent. Or 2nd call + 2nd fraction, just guessing. If your software requires entry of info from the 'money box' (race frequency, earnings, in-the-money ratio, etc), then your mystery pace-class fig could be some pace velocity modified by an earnings/frequency class measure. If there is no such 'money box' info in your input, my guess is it is simply some kind of pace-related info.
RDSS has numerous Pace AND Class related info (Class as derived from Average Purse Value, ITM consistency, Earnings per Start and also a weighted final odds formula from recent races).
You could propose a race or races for someone to put up some pace/class/BL or Rx oddsline readouts from in RDSS and see how they compare to your readout. RDSS may be as effective (and likely better
).
If it's not a Sartin product, well anything goes! Sartin's name has been 'borrowed' to sell many ideas over the years.
Again, welcome to the forum, and good luck. (Consider using the Google Site Search tool on the site menu bar for various research).
Ted