大方廣圓覺修多羅了義經略疏 2.1361

2.1361

宗是頓教，事具漸門。既頓漸俱收，則遲速皆益，入與不入總可留心，故正宗中分上、中、下。

Its principle is the sudden teaching, while in practice it possesses gradual gates. Since it gathers both sudden and gradual, both the slow and the swift benefit from it; whether entering or not yet entering, all may apply their minds to it. Hence within the main body it is divided into superior, middling, and inferior.