Screen space
The last stop on the transform chain: coordinates in actual pixels on the render target, plus a depth value as a souvenir of the third dimension.
The viewport transform scales normalised device coordinates to the width and height of the window. Where the origin ends up depends on the API: top-left in Direct3D and Vulkan, bottom-left in OpenGL — yet another source of upside-down images. HUDs, cursors and text are often drawn straight into screen space, without detouring through a 3D world at all.
"Screen-space effects" are a family of their own: SSAO, screen-space reflections and motion blur work only with what is already sitting in the frame's buffers. That makes them cheap and independent of scene complexity, but also blind to anything off-screen or hidden behind something else — which is why reflections flicker at the edges when the camera turns.