What happens if I chose CPU, but with a GPU specified in settings? I have Lenovo Legion Y7000 with a Graphics card of 1060. In the System, Settings that you can chose what you want to render with CPU or GPU, I chose GPU, then on my right hand side are those tabs are a Camera Icon, under if you look for it, it says device. Under Render, Animation, and Audio. There's a drop down menu, choices between CPU and GPU. What happens if I chose CPU, but in my settings its as GPU.
The option in the preferences controls which OpenCL/CUDA compute device will be used when the GPU option is chosen in Properties > Render.



Why this is arranged so confusingly is due to history. Originally cycles only had CUDA support, and so was a natural assumption that the "compute device" chosen in the preferences was a GPU (or GPUs). Since OpenCL can work on CPUs, it's possible you could end up using OpenCL on your CPU when GPU is selected in Properties > Render.