प्रतिपादिक is the sense (epistemological) of a word. The gender of a word's sense (प्रतिपादिक) can imply i) अलिंग (gender-less category-indicating words/common-gender) ii) नियतलिङ्ग (decisive gender i.e. masc./fem.) iii) अनियतलिङ्ग (indecisive gender or words where gender doesn't make sense/neuter-gender). प्रतिपादिक can also indicate "number".
The rule says that if a प्रतिपादिक (in nominative case) represents multiple genders or numbers (आधिक्य) then प्रथमा is implied.