Personalized Difficulty and Topic Authority can be found in Topic Inventory and Reflect.
Personalized Difficulty is a metric that’s unique to your site and its contents. The value ranges from 1 to 100 — the higher the number the harder it is for you to rank. It’s calculated based on your site’s coverage of a topic, how well it performs (both currently and historically), and other factors.
Use this to determine how much effort an opportunity to create/update requires.
1 to 20 — optimize the existing page (assuming the topic matches what the page is about), otherwise create a new page.
20 to 35 — optimize the page as above and create/optimize a page or two of supporting content
35 to 50 — optimize the page as above and build out/optimize your existing cluster.
50 plus — most likely you don’t have a cluster so you’ll need to build out this foundation.
Topic Authority can be considered as the difference between (Keyword) Difficulty, a generic metrics that applies to everyone, and Personalized Difficulty, which only applies to your site. The greater the difference between the two metrics, the bigger your authority.
Look for situations where you have a Topic Authority of at least 5 to 10.
Think of it as a probability indicator — the greater your authority, the more likely you are to succeed at optimizing an existing page or creating new content. It also reveals your competitive advantage as it means you have an easier time ranking for a topic than your competition.
Use Personalized Difficulty with Topic Authority — prioritize opportunities that don’t require much effort (low Personalized Difficulty) where you’re most likely to succeed (high Topic Authority).
Overlaying these personalized metrics on top of your standard quick wins (ranking on 2nd page of Google) approach can yield to more predictable success.
Learn more in What is Personalized Keyword Difficulty and Why You Need It