成唯識論了義燈 5.164

5.164

答：以影顯故。上下二品，俱說加行及解脫道，故於中品略而不說。又斷中品同一作意，雖有四道更不別說。

Answer: It is through implied indication. For both the upper and lower grades, the prayoga and vimuktimārga are mentioned, and therefore they are omitted for the middle grade. Furthermore, cutting off the middle grade employs a single manaskāra; although there are four paths, they are not explained separately.