The sense which is the cause of application of the suffix is called प्रकृति. The rule says that the word that results from the suffix that has undergone लुप् should confirm with the प्रकृति. In other words, प्रकृति would supercede the post-लुप् state. For example, पञ्चाल अञ् - पञ्चाल (masc. pl. in one sense or "जनपद" masc. sl. in another sense) which results in पञ्चालाः or पञ्चालः.