Duel tiers are the amount of Bolts put on the line in each Duel (or match).
Every Duel awards 1,000 XP points + Tier XP Star bonus. However, the higher the Tier (measured in Bolts played), the more experience (XP Stars) gained.
You will gain the same amount of XP Stars even if the game is lost. So this is a great reason to test your skills and pick the high Duel tiers!😉