What does (in)definite mean? It’s uniqueness, familiarity and existentiality

INTERNET SOURCES

1. 
   https://www.wallstreetenglish.com/exercises/the-complete-guide-to-definite-and-indefinite-articles
   

2. 
   https://www.englishclub.com/grammar/determiners-articles.php
   

3. 
   https://www.lewolang.com/english-grammar/16/definite-and-indefinite-articles
   

4. 
   https://www.richtmann.org/journal/index.php/ajis/article/view/10201/9833
   

5. 
   https://www.usinggrammar.com/english-grammar/definite-indefinite-articles.php