Yep its the class allowance used to calculate going allowances. Mordin used 5 lengths per second in that book, so for example he used a 20 length difference per mile between Class A and H.
He also corrected for 2yo/3yo depending on the time of year and used a fillies allowance.
Back in the day when I had time to do my own speed ratings I checked those allowances by comparing different class races, run over the same distances at the same meetings and found they largely stack up.