瑜伽師地論略纂 16.37

16.37

不淨三相中，「等流」者，與糞穢為等流相。第一、即體是糞穢也；第二、是糞穢等流；第三、但有四大及造色所依、能依也。

Among the three aspects of impurity, "naturally flowing" means having an aspect that is a natural outflow of excrement and filth. The first is that its very substance is excrement and filth; the second is the natural outflow of excrement and filth; the third is merely having the fundamental elements and derived form as basis and supported.