Based on the ratings the users get from challenges, they're given ranks for the season. For season, the user falls into tier based on his rank.
For the current season, Seaside Season the tiers are as follows
tier 1 - COMMODORE - ranks 1-10
tier 2- FIRST MATE - ranks 11-25 and so on
This informations is shown to users on the tiers for seasons (1st tab in the consolidated popup).