It's for a team render(animation) started through the render queue. It was already started.
"Custom Number of Render Threads" under Preferences -> Renderer -> Team Render" I changed it here, but after reading your post I think this may only control the number of threads if my machine is used as a render node for a job started on another PC.
I'll have to do some more tests once this render is complete.
For now, I simply untick my local machine from the team render machines window, and that works just fine, it removes my workstation from the rendering but it continues rendering on the other nodes, and when I'm done I tick the checkbox again, and it resumes rendering.