Proficiency Bonus

Factions have a proficiency bonus determined by tier, as detailed in chapter 1. The bonus is used in the rules on ability checks.

Your faction's proficiency bonus can't be added to a single die roll or other number more than once. For example, if two different rules say your faction can add its proficiency bonus to a Wisdom ability check, you nevertheless add the bonus only once when you make the save.

Occasionally, your faction's proficiency bonus might be multiplied or divided (doubled or halved. for example) before you apply it. If a circumstance suggests that your faction's proficiency bonus applies more than once to the same roll, you still add it only once and multiply or divide it only once.

By the same token, if a feature or effect allows you to multiply your faction's proficiency bonus when making an ability check that wouldn't normally benefit from its proficiency bonus, you still don't add the bonus to the check. For that check your proficiency bonus is 0, given the fact that multiplying 0 by any number is still 0. For instance, if your faction lacks proficiency in the Intelligence ability score, it gains no benefit from a feature that lets it double your proficiency bonus when it make Intelligence checks.

Expertise

Certain features, such as the Association Expertise feature, let your faction double its proficiency bonus with a given ability score, granting expertise in that ability score. As usual, you can only gain expertise in an ability score once.