俱舍論記 24.33

24.33

「言上流者至方般涅槃」者，謂生色界二生已去般涅槃者，總名上流。此唯上流，不下流故，名為上流。異生之人雖有上流，亦下流故，不名上流。

Regarding 'That called "upper stream"... only then enters parinirvāṇa': those who are born in the form realm and enter parinirvāṇa after two or more rebirths are collectively called 'upper stream' (ūrdhvasrotas). Because they stream only upward and never stream downward, they are called 'upper stream'. Although ordinary persons may stream upward, they also stream downward; hence they are not called 'upper stream'.