The Audience Interests: Cross-Browsing Behavior and Relevancy Score are two metrics that help identify valuable websites that target audiences visit.
Cross-Visitation: Average percentage of users who visited both this website and the analyzed website on the same day.
"Average" is the aggregation of unique users that visited both websites that month, averaged per day. Cross-Visitation data goes back to December 2017.
Relevancy Score: The relevancy score is based on a joint audience; i.e., the score between pairs of sites is calculated based on users who visited both sites relative to the size of these sites.
A simple mathematical explanation (we are doing something close to) can be the ratio between the users who visited both sites to the users who visited at least once.
In addition, there are few more considerations taken into the score, like the size of the two sites (so that amazon wouldn't get some small sites as the most relevant, for example), and the intent of the user in certain situations (i.e., if users who visited sites A and B, always starts with A the score between them will not be symmetric).
Relevancy score data goes back to June 2017.
Note: Cross-Browsing Behavior and Audience Interests data is based on unique visits.