The term "engagement" that is seen in mailing/campaign reports refers to a formula involving campaign recipient actions (opens, clicks, forwards, unsubscribes, etc.) divided by the number of recipients of that campaign.
This score is commonly used to gauge recipient interaction for the campaign as a whole and to identify campaigns that may not be as effective.
You can manage this scoring structure for your account in Account Management > General Settings > Settings.
1. It is important to note that the engagement score is an index or summary of combined actions, and not measured on a set scale (i.e. on a scale of 1-10).
2. It is also important to understand how differing audiences or differing campaign types (i.e. informational vs. promotional) can return very different measurements of engagement. Compare "apples to apples" when using the mailing engagement score to draw conclusions of your campaign success.
3. There is no universal "right" or "recommended" engagement score, because not all accounts use all of the tracking features included in the formulas. But because the formula is the same for every campaign within an account, the engagement indexes are useful for comparisons to see which campaigns are performing the best.