瑜伽師地論略纂 14.20

14.20

論云「如是六位，復開為八，若六平等」者，謂初二淨不淨為二，堅不堅為二，故八也。

The treatise says: "These six stages are again opened into eight, if the six are equal." This means that the first two—pure and impure—make two, and firm and infirm make two, hence there are eight.