Could you please tell me which grammar rule these two "인"s belong to?

1. 인류의 가장 오래된 조미료인 백색의 작은 금,소금

2. 생명을 유지하는 데 필수적인 물질로 Perhaps the grammar here is 필수+적인?

Could you please tell me which grammar rule these two "인"s belong to(in these two pages)?

https://www.koreanwikiproject.com/w/index.php?title=Category:Grammar&pageuntil=%E3%85%87%0A%28%EC%9C%BC%29%EB%A9%B4+%EC%A2%8B%EA%B2%A0%EB%8B%A4#mw-pages

https://www.koreanwikiproject.com/w/index.php?title=Category:Grammar&pagefrom=%E3%85%87%0A%28%EC%9C%BC%29%EB%A9%B4+%EC%A2%8B%EA%B2%A0%EB%8B%A4#mw-pages

You can comment in Korean, I can use a translator. 너무 감사합니다