Hi Nicolas,
Thanks for reaching out to us here–great question! When reviewing your Event Aggregator settings, if you’re wanting to ensure that edits to your imported events are preserved on subsequent imports, you’ll likely want to choose the third option for event update authority: Preserve the most recent change in each event field, whether that change occurred on my site or at the original source. This selection will ensure that Event Aggregator favors the most recent change to any field on a given event, regardless of where that change was made. To use your example, if you were to import an event from Facebook that did not have an event description, you could add a description to the event on The Events Calendar after your import, then import the event again–since no changes had been made to the description field on the Facebook event after you’d edited the description on the imported event, the field wouldn’t be changed on subsequent imports.
I hope this explanation helps to clarify things a bit! Thanks again, and please don’t hesitate to follow up if you should have any other questions! 🙂