The niṣṭhā ta is optionally changed to na after 1. nuda 2. vida 3. unda 4. trā 5. ghrā and 6. hrī,
Karika:-
वेत्तेस्तु विदितो निष्ठा विद्यतेर्विन्न इष्यते ।
वित्तेर्विन्नश्च वित्तश्च भोगे विन्नश्च विन्दते ॥
Thus नुन्नः or नुत्तः, विन्नः or वित्तः, समुन्नः or समुत्तः, त्राणः or त्रातः, घ्राणः or घ्रातः, ह्रीणः or ह्रीतः ॥
With regard to ह्री, the न change was not ordained by any rule, and so It is an aprapta-vibhasha. With regards to others, the न change would have always taken place by 8.2.42 and 8.2.43, this makes it optional.
The root विद विचारणे of Rudhadi is to be taken here, and not the other विद roots. Thus the following : Karika
The Nishtha of विद्-वेत्ति of Adadi class is विदितः; (2) of विद्-विद्यते of Divadi class is विन्नः only; (3) of विद् of Tudadi is विन्नः; (4) of विद् of Rudhadi are both वित्तः and विन्नः ॥ The Tudadi विद् has also the form वित्तः in the sense of भोग by 8.2.58.,