Hi TJ,
These are external activities, used by end users to generate campaign responses via manual file upload. It is used when there is a need to create a campaign member in CRM via campaign integration, where that person has "responded" to a campaign via some channel that Eloqua does not track or capture - such as offline trade shows. These are not the same as standard contact activities, such as email open, email clickthrough, email send, etc.
To preface, there are REST endpoints for retrieving these (/data/activities), but it involves making one API call per contact, per activity type, and retrieves data for up to one year. This is the endpoint the application uses when you open the contact record, and navigate to the Activity Log tab, and it's not efficient for compiling email metrics. For example, if you wanted to know how many email opens there were for Newsletter B, you'd have to query every contact in your data base for the activitytype of emailopen.
The more efficient process would be to follow the asynchronous Bulk 2.0 workflow to create an export definition that defines what activity type you want, what fields, and other filter conditions such as activity date or asset id. The export definition is then "synced", which provides a transactional sync id. This can then be polled to query the completion status. Once the export has finished generating, you can retrieve the data in either JSON or CSV. The full Bulk 2.0 documentation guide goes through this in far more detail.
Regards,
Bojan