What does the % inside a cluster mean?

The URL overlap percentage refers to the percentage of shared URLs between the main keyword (the title of the cluster) and the other keywords in the cluster.

If the overlap is low, it may be more effective to write a separate article for the keyword with the low overlap and link it to the main cluster article.

This can help to ensure that the content for each keyword is organized and focused, and it can also help to improve the overall structure and navigation of your website.