Talk:Survival/@comment-222.167.129.142-20140512111208/@comment-84.193.142.189-20140625142012

When the max level of the enemies has been reached, you will not get a higher chance. I think from 25 onwards, the enemies are at their highest level. In regular Survival missions, it's best just to stay until 20 or 25, and then start over. In the Void, that's not the case, as you spend a key upon extraction.

Survivals have three tiers of difficulty IIRC. 5 and 10 are tier 1 difficulty (and rewards), 15 is tier 2, and 20 is tier 3. Afterwards, the rewards start over, but the difficulty remains at tier 3 (highest difficulty level stated in node description)

I could be wrong, but this is what I think is the mechanic of difficulty levels in Survival.