妙法蓮華經玄義 2.77

2.77

位妙者，謂三草位、二木位、一實位，妙行所契，故言位妙。

The subtlety of stages: this refers to the stages of the three grasses, the stages of the two trees, and the stage of the one real. Because they are what subtle practice accords with, it is therefore called the subtlety of stages.