Viewport "use_hdr_2d" documentation point unclear

**Your Godot version:**
4.3
**Issue description:**

in the description of "use_hdr_2d" there is the following sentence: "Additionally, 2D rendering will take place in linear color space and will be converted to sRGB space immediately before blitting to the screen (if the Viewport is attached to the screen)"

I can't find reference anywhere in the documents about the concept of attaching a Viewport or SubViewport to the screen. The closest I have found is in the 3.2 docs Viewport.set_attach_to_screen_rect. Please could this point be clarified? What defines a viewport as being 'attached to the screen'? thanks!

**URL to the documentation page:**
https://docs.godotengine.org/en/stable/classes/class_viewport.html#class-viewport