大方廣圓覺修多羅了義經略疏 2.130

2.130

遇於勝教皆成，即明無別定性。反明不遇勝緣，則隨所聞成小，文無者略也。由此名不定性，故上標頓漸。

"Encountering the superior teaching, all achieve it" explains that there is no distinct, determinate nature. Inversely, it clarifies that if one does not encounter superior conditions, one achieves the small [vehicle] in accordance with what is heard; that the text lacks this is an abbreviation. Because of this, it is called the indeterminate nature; therefore sudden and gradual were indicated above.