Tier 1 and Tier 2 survival question

I have ADD and reading Warframe wiki can be me reading the same sentence 8x to try to absorb it.

In reference to tier 1 and tier 2 rewards for Conjunction survival rewards… am I understanding that 0-20 minutes is tier 1 and 20-40 minutes is tier 2?

If so, what is 40-60 minutes? bc the rewards only describe tier 1 & 2 for Voruna parts.

Thanks