This tab shows all of the devices in the central processing unit (CPU) of your computer. This means this default device differs according to the computer's hardware configuration. The default device selection picks the first available CUDA device is installed otherwise, CPU is selected. A render session is the rendering progress in a Raytraced (Cycles) viewport. The device selection grid is for selecting the device to render new render sessions with. This can be used to lessen the stress on the GPU, especially useful when rendering on GPU where the GPU is also the display driver, meaning a monitor is attached to that card, and the operating system is using that to show everything on it.
The time in milliseconds to wait between each sample pass (one pass over the entire image, sampling each pixel once).
The Cycles options manage the device used for the Raytraced display mode and Rhino Render.
