Potential Accumulated Impressions (PAI) is the maximum number of impression that you could have reached during a campaign, if all the posts of all the influencers had been seen by all their followers.
Why do we use PAI
Impressions are not available from Instagram, so we use PAI as Potential Impressions. So it is how many times a post was viewed potentially. It is simply the number of followers multiplied by the number of posts.
How we calculate PAI
Followers * Number of posts of all influencers in the campaign
Why it's important
It represent the maximum number of impression of your campaign. Please note that it doesn't represent the Reach.