TLDR; For general networking optimization reasons, you must manually replicate the spawning of every Feedback Event Factory just like you would manually replicate the spawning of a particle effect or sound effect.
Hey, bok! The “FET_All” setting simply means that the event has no filter and will play for ALL clients that spawn the Feedback Event Factory.
As a different example, if you chose “FET_Owner”, the event would only play for the owner assigned to it, even if you spawn the Feedback Event Factory on all clients.
The most common use case for stuff like this is when you have one effect you want to play in first person for the owner and then a separate effect to play for everyone else (e.g. a muzzle flash effect that has a lot of detail in first person with non-spatialized audio, and then a different less detailed effect with spatialized audio).
Hopefully that makes sense. This is a more advanced feature that I typically use when fine tuning effects for 1st person (Owner) versus 3rd person (NonOwner).