What do the attribution-related event_properties related to in the events API?

Hi everyone!

In the Events API, under event_properties it appears there are some attribution-related objects/values that are returned for certain events. Here are the relevant pieces I’m referring to: 

1. Why does this information only exist for certain events?
2. Is there any documentation on the relevance of these values? How can this attribution information be used?

I don’t see any documentation about attribution info in the API. If I’m just missing it, please let me know. 

Thanks!