The visarjanīya is substituted for ru called ra (and not any other ra) before the Locative -7th case plural case-affix su,
Thus पयःसु, सर्प्पिःषु, यशःसु ॥ The word सुप् is here the Locative Plural affix. Though the र् would have been changed to visarga by the last sutra also; the making of this a special sutra is for the sake of niyama. That is, only रु becomes visarga, and not any other र् ॥ Thus गीर्षु, भूर्षु, when the र् is not रु ॥ In पयस् &c, the स् becomes रु by 8.2.66.,