Case Study 2: Investigating Metric Spike - Email Engagement Analysis

In the "action" attribute there are 4 actions present: email_open email_clickthrough sentweeklydigest sentreengagementemail I need to know what does emailclickthrough and sentweekly_digest stand for? if an email has been opened it will fall under the "emailopen" category. Similarly what does emailclickthrough mean? Please elaborate. Secondly, a user sends the weekly digest? Or do they receive it? If they receive it then only they will open an email. Here it's not clear why if a user sends emails for reengagement and weekly digest, we have a category for email open? What kinds of email does the user open?