When you go to examine one of your survivor’s profile, you can see the number of collected Class Tokens in the lower right corner. Each class has their own tokens that can be used to unlock and upgrade traits of the survivors that are the same class.

You can get Class Tokens from various places such as:

• Challenge Stars rewards
• The Distance rewards
• Daily Quests rewards
• Trade Goods Shop
• Radio Call Screen if you decide to reject a survivor

Once you have fully upgraded all traits of a survivor, you will have the opportunity to promote them to the next rarity level.