Quality Settings - how much is half res?

I've looked at the documentation of quality settings (https://docs.unity3d.com/Manual/class-QualitySettings.html) but it doesn't say anywhere what half-res is.

The intuition tells me it would correspond to 4K -> 2K, but that is in reality a quarter of the original resolution.

Which one is it?

 

Construct an experiment and find out!

 

Experiment:

create a 4K image with diagonal patterns
add 4K image to a camera overlay canvas
enable mip maps on the image asset (otherwise quality settings won't work)
set max size on image asset to 2K (because my screen isn't 4K)
add resolution settings (960x540, 1920x1080) in unity game view
note: simply highlighting a quality setting in editor will update the game view (if mip maps is enabled on the texture)
look at image with full res, half res and quarter res in both resolution settings and scale the game view accordingly.

Results:

half res didn't affect the appearance at 960 x 540
quarter res did affect the appearance at 960 x 540

Conclusion:

half-res must be both width and height halved, thus a quarter of the resolution.

 

I hope it will be added to the documentation for future developers.