Tiers refer to the base level of difficulty of a mission (Low, Medium, High, Very High, Extreme) and its level of boosts and equipment dropped, outside of Exotic equipment from bosses.

Exotics are not confined by Tiers.

Mission's difficulty directly corresponds with the tier level of loot dropped within it. Elite versions drop the same loot as the non-elite versions, but the difficulty shown is that of the next tier level up.

For example, Fire Forest, a low difficulty level mission, would only drop tier level 1 loot. Elite Fire Forest, which still drops tier level 1 loot like non-elite Fire Forest does, is considered to be a Medium difficulty level mission, the same difficulty level given to Fungus Cave and Vanquished.