What units is the Rect Transform Pos when using Screen Space - Overlay?

I’m guessing that those are in world space units but then the question becomes how far the virtual “overlay” is from the viewpoint? Or are they in pixel units?

Looking at the scripting documentation, it appears that “Screen Space - Overlay” is not even a mode in the scripting. Only “Screen Space” and “World Space” are possible render modes.

Rect Transform is in world units. If you look at the inspector component of the Canvas you can see that Screenspace - Camera gives you the options to set a plane distance. You can set it via a reference to Canvas.planeDistance in code, too. You can also set the render mode via script to Overlay or Camera. Unity - Scripting API: RenderMode

What are you trying to do exactly?