na-adjective, noun which may take the genitive case particle 'no'
absolutely essential; indispensable; vital
Stronger than 必要 alone; emphasizes that something is not just needed but cannot be done without. Often used in formal or explanatory contexts.
水は生命にとって必要不可欠だ。
Water is absolutely essential for life.
この
This document is indispensable for the application.