成唯識論演祕 1.638

1.638

略為二釋：一云，體性是常，唯實上有，名常實轉。二云，恒常唯在實，句上轉名為常也。

Briefly, there are two explanations: One says that its intrinsic nature is permanent and it exists solely upon substance, so it is called 'constantly functioning in substance'. The other says that it permanently and solely functions upon the category of substance, hence called 'constantly'.