The affix ghañ is optionally used after the verb vṛ 'to choose' when pra is in composition and the word so formed means 'a sort of covering'.,
As प्रावारः or प्रवरः 'a cloak, a veil'.
Why do we say 'when it means a covering'? Observe प्रवरागौ ।,
वृणोतेर् आच्छादने
Adhyāya 3 · Pāda 3 · Rule 54