妙法蓮華經玄義 2.83

2.83

利益妙者，謂果益、因益、空益、假益、中益、變易益，猶如大海能受龍雨，故名利益妙。

The subtlety of benefits: this refers to fruit benefit, causal benefit, empty benefit, provisional benefit, middle benefit, and transformation benefit. Just as the great ocean can receive the dragon's rain; hence it is named the subtlety of benefits.