Use SingleThreaded for internal debugging. It uses only a single thread to simulate RenderingThreadingMode.MultiThreaded.
In this mode, the main thread processes the application's high-level code through an internal class GfxDeviceClient that issues an intermediate representation of all the required graphics commands.
Those intermediate graphics commands are forwarded to a separate internal class GfxDevice[Platform] that converts them into the platform's low-level graphics API commands.
On some platforms you can test this mode by passing the command line argument -force-gfx-st to an application built with BuildSettings option BuildOptions.Development.