Campaigns

To my knowledge the data here is the same as the campaigns tab in the explorer.

Campaigns are emails that go to Microsoft tenants, not necessarily just your tenant but any message you see here will only be from your tenant. Microsoft does not give away too much about how campaigns are identified but does state it looks at the sender source, the recipients and their role or industry, and the payload.

Campaigns are awesome because they track all messages in the campaigns, including replies or forwards. This allows you to easily delete the original message and the copies that may have been sent to other mailboxes by your users.