The affix ac is used after the verbal roots when śam 'well' is in composition and the word to be formed denotes a name.,
As शम् + कृ + अच् = शङ्करः lit. 'who does good'. So also सम्भवः संवहः । All these are proper nouns.
Why have we repeated the word dhatu in this aphorism, when it was understood by anuvritti from sutra 3.1.91? To this the reply is, that the repetition of the word dhatu shows that there is prohibition of the affix ट, that comes after the verb कृ in certain senses 3.2.20, when the word शम् is in composition. The affix अच् will be employed after the verb कृ and not the affix ट्, when शम् is an upapada, even when the sense to be indicated is that of cause, habit &c. 3.2.20. The result is that the feminine of शङ्कर will be शङ्करा, and not शङ्करी, which, had the affix been ट, would have been the proper feminine (See 4.1.14). शङ्करा means 'a female-ascetic', 'a vulture' and 'one having the habit of doing good'.,