瑜伽師地論略纂 9.53

9.53

論云「合會無常」，云即以前三種無常，觀現在法與此三種無常和合故，名合會無常。即前樂受於現在世流入此三無常也。

The treatise mentions "the impermanence of confluence," stating that one contemplates present dharmas through the preceding three kinds of impermanence as being in confluence with these three kinds of impermanence; hence it is called "the impermanence of confluence." That is, the aforementioned pleasant feeling flows into these three impermanences in the present life.