What is "Event Driven Loader" and What is different with "Asynchronous Loading Thread"?

Hello, I’m currently digging into ASync Load.

I found codes about “Event Driven Loader”(EDL) in FAsyncLoadingThread class.
I can’t understand what it it… There is no document about it…
What is “Event Driven Loader” and What is different between EDL and “Asynchronous Loading Thread”?