Hi all,
Though external activities can totally be used separate from Close Loop Reporting, the documentation is typically closey aligned with CLR. You'll find lots of documentation around CLR. A quick, good one is EloquaU Power Hour: Closed-Loop Reporting.
The basic premise of external activities is that you can upload a list of contacts and generate an external activity (an activity or response that Eloqua does not natively see). These activities can be associated to a campaign or not. Then similar to activities that Eloqua tracks (e.g. email opens, clicks, form submits, etc.), that activity shows up on the Contact's record, can be used for segmentation and personalization and can be seen in reporting.
Hope this helps!
E