The affix phak comes in denoting a gotra (Patronymic) descendant after the words 1. śaradvat 2. śunaka 3. darbhā when they denote the descendants of bhṛgu , vatsa and agrāyaṇa , respectively. ,
Thus शारद्वातायनः = भार्गवः otherwise शारद्वतः (formed by अञ्) शौनकायनः = वात्स्यः, otherwise शौनकः, दार्भायणः = आग्रायणः, otherwise दार्भिः, (formed by इञ् 4.1.95).
The words शरद्वत् and शुनक belonging to बिदादि class would by 4.1.104 have taken the affix अञ् in denoting Gotra, the present sutra prohibits that in certain cases and enjoins फक् instead.,