The affix vuñ comes in the sense of 'nature or action thereof' after a polysyllabic stem whose penultimate letter is y and whose penultimate syllable is prosodially heavy.,
The word उपोत्तमं means the last syllable but one in a word of three syllables or more. That word whose penultimate syllable is गुरू 'heavy', is called गुरूपोत्तमम् : the योपधा means 'having penultimate य्'.
Thus रामणीयकम् from रमणीय, वासनीयकं from वसनीय ॥
Why do we say \the penultimate letter being य्\? Observe विमानत्वं from विमान ॥
Why do say गुरूपत्तिम \the penultimate syllable being heavy\? Witness क्षत्रियत्वम् ॥
Vart:- Optionally so after the word सहाय, as, साहायकम् and साहाय्यम् ॥
The words उपोत्तम् and उपधा should be distinguished: the first means penultimate syllable (implying thereby the word is of more than two syllables), and the second means penultimate letter.,