The affix ṇini comes after a verb, in composition with a word ending in a case-affix.,
The word व्रत means a 'vow' as regulated and ordained by the Scriptures. This condition 'when vow is expressed' applies to the completed word, that is to say, when the complete word made up of the root, the upapada, and the affix, denotes a vow. As स्थण्डिलशायिन् 'the vow of sleeping on bare ground (and nowhere else) or a person who has taken this vow'. अश्राद्धभोजिन् 'who has vowed not to eat during the performance of a Sraddha ceremony.
Why do we say 'when vow is to be expressed'? Otherwise there is no composition. As स्थण्डिले शेते देवदत्तः 'Devadatta sleeps on bare ground'.
This sutra is also an exception to 3.2.78 applicable when habit is not meant, or when genus is to be expressed.,