What is the data element selection for in the mapping table?
I was watching this video on the ACDL and how it is consumed and mapped in Launch.
Adobe Client Data Layer - Event-Driven Data Handling | Adobe Data Collection
What wasn't explained was what the data element selection, at the top of the mapping table, of "event message event" is for and how it is leveraged.

Does anyone know what this selection does? How this "event message event" data element would work? Also I'm looking for more documentation or videos on how to work with the EDDL and how it integrates with Launch (tags).
Thanks!