Audience Reachability indicates the percentage ratio of followers among your audience who can see an influencer's posts in their feed.
Audiences who follow less than 1,500 accounts are considered reachable, as the likelihood for them to see the post in the newsfeed is higher.
Below you can see the comparison of 'Average' and 'Excellent' Audience Reachability. In the first case, the user's audience is slightly less reachable, since about 15% of it's followers follow more than 1,500 accounts.
Audience Authenticity reflects the percentage of total authentic audience. It takes into account real people and influencers, as both types are considered to be real profiles.
There are ranks that will indicate whether audience is very good, average or could be improved. Each metric has a benchmark for similar size influencers.