How spaces works in unity?

Indeed, there are several spaces in the world of the 3D and computer graphics.

A space is defined by :

   1. An origin = a reference point

   2. Unit vectors, defining the "directions" of the possible dimensions

The main spaces are:

   • World space :

        Origin = (0, 0, 0)

        Unit vectors = { (1, 0, 0), (0, 1, 0), (0, 0, 1) }

        3D GameObjects without a parent are defined in the world space

   • Local space :

        Origin = position of the parent gameObject

        Unit vectors = axes of the parent gameObject, according to its rotation

        3D GameObjects with a parent are defined in the local space

   • Screen space :

        Origin = bottom-left corner of your screen (0, 0)

        Unit vectors = { (1, 0), (0, 1) }, where one unit represent one pixel of your screen. The top-right corner of your screen has the following coordinates (Screen.width, Screen.height)

        Input.mousePosition is defined in the Screen space

   • Viewport space :

        Origin = bottom-left corner of your screen (0, 0)

        Unit vectors = { (1, 0), (0, 1) }. The top-right corner of your screen has the following coordinates (1, 1)

        It's the normalized position of an object on the screen according to the screen dimensions

As indicated in the documentation

ScreenPointToRay casts a ray starting on the near plane of the camera and going through position's (x,y) pixel coordinates on the screen.

Calling .origin will return a world space point liying in the near plane of the camera.