The World Tier that you have access to depends on your performance in quests/missions. The better you play, the more World Tier experience, allowing you to unlock the next Tier. If you would prefer an easier tier, you can select it from the menu, but you will not be able to generate experience to get to your next step in World Tier difficulty.
You also lose World Tier experience when you die, adding risk and ensuring people do not simply spawn spam their way through the World Tiers. If you want to progress, you will need better loot to help you survive. As you climb through the Tiers, min/maxing and properly performing well-thought-out tactics with your cooperative buddies will become more important.
Loot Drops – An Interesting element about World Tiers is that they will allow you to get loot drops that are higher than your character level, and you will be able to use them in that World Tier, but not in a lower one. As you climb through the Tiers, this additional loot drop level will continue to increase.
Scaling based on the player count – Outriders’ difficulty level will also be scaled based on player count. If you are playing with buddies, things will be more difficult than if you were playing alone