Real Time Rendering
This is the default render engine. As the name suggests, it renders your 3D scene in real time, allowing for fast performance, animation, and smooth interactivity. Scenes created with real time should be viewable on most devices with no performance issues.
AR and VR scenes aways use real time rendering.
Ultra Render
Ultra Render is a GPU Path Tracing engine that is capable of producing photorealistic renders. The engine traces paths of light beginning at the camera lens and ending where the path meets a surface and calculates all the light rays contained in that path. It then processes that light in the way a human eye would, and produces a render in which the lighting, shadows, and reflections appear as they would in the real world.
This rendering process is more expensive for computers and requires high graphics capabilities. It is best for rendering still images.
Ultra render is not available for AR or VR, and will not perform on mobile devices.
Ultra Render Tutorial