Doubt regarding CS 2, Question E

Hi In Case Study 2, Question E, I didn't understand what "email engagement metrics" is? Is it the same as Question A and D i.e. weekly engagement? Also, in (https://trainity.sleekplan.app/feedback/79963), in Case Study 2, Point 5, the mentor said that we have to join events table with emailevents table on userid. I didn't understand why. The email event types and the dates of events are provided in the email_events dataset, then why we need to join the two tables?