A sārvadhātuka affix beginning with a consonant, optionally get the augment iṭ after the roots 1. tu 2. ru 3. stu 4. śam and 5. am,
The root तु (Adadi 25) means 'to increase', रु (Adadi 24) 'to make a sound', स्तु (Adadi 34) 'to praise', शम् 'to be satisfied', and अम् 'to go'. According to Kasika तु is a Sautra dhatu. Thus उत्तौति or उत्तवीति, उपरौति or उपरवीति, उपस्तौति or उपस्तवीति, शाम्यध्वम् or शमीध्वम्, अभ्यमति or अभ्यमीति ॥ शम् and अम् can then be followed by a consonant beginning sarvadhatuka affix, when they lose their Present character (vikarana) as a Vedic anomaly (bahulam chhandasi).
The Apisalas read the sutra as तुरुस्तुशम्यमः सार्वधातुकासुच्छन्दसि ॥ This will then become a विधि rule for the Vedic forms. The word सार्वधातुका is here exhibited in the feminine.
The repetition of 'sarvadhatuka', though its anuvritti was present is for the sake of stopping the anuvritti of पित्, and this rule applies to अपित् affixes also, as सुवीत, शमाध्वम् ॥,