Hello,
There is not a official definition of global user engagement, but there is a blog part I found may answer your question.
“Bing also considers how users interact with search results.” How does Bing do this? Bing says “to determine user engagement, Bing asks questions like: Did users click through to search results for a given query, and if so, which results? Did users spend time on these search results they clicked through to or did they quickly return to Bing? Did the user adjust or reformulate their query?” In fact, Bing Webmaster tools provides these analytics and insights into how users interact with your webpages. Bing can use those insights for ranking purposes.
I will update if I get more insight here.
Hope this helps.
Regards,
Yutong