The guṇa is substituted for the final ik vowel of a aṅga (stem) before the affixes called sārvadhātuka and ārdhadhātuka -- tiṅśitsārvadhātukam [[3.4.113]] etc.,
As तरति, नयति, भवति, कर्तृ, चेतृ, स्तोतृ ॥ Why sarvadhatuka and ardhadhatuka affixes only? Observe अग्नित्वम् अग्निकाम्यति ॥ For had the sutra been सङि then the rule would have applied to affixes like सन् काम्यच &c which go to form Denominative verbs. सङ् includes all affixes beginning with सन् and ending with महिङ् ॥ If the sutra had been प्रत्यये, then the rule would have applied to the affixes like त्वम् &c. To exclude these cases, the two words sarvadhatuka and ardhadhatuka are used. For exceptions See 1.1.4, 1.1.5, 1.1.6.,