妙法蓮華經玄義 2.76

2.76

行妙者，謂增數行、次第五行、不次第五行，智導行故，故言行妙。

The subtlety of practice: this refers to the practice of increasing numbers, the graduated five practices, and the non-graduated five practices. Because wisdom guides practice, it is therefore called the subtlety of practice.