What is Client 0 supposed to be? Why is it there?

Hello Friends,

Today’s question is simple, though the answer might not be.

I am binding an event on a key press in blueprints, the event then fires whenever the client detects pitch over the microphone. If I print a debug string on the key press, it fires on Client 1, as expected. If I print a debug string from the event, I get a debug string printing from both Client 1, as expected, and Client 0, which to my knowledge does not exist.

What is Client 0? Why is it only showing up in this one case? Why is it even a possibility?