What do setting CGFontRenderingFontSmoothingDisabled from defaults actually do? I was trying to make fonts on external non-retina display look good and I was playing with setting CGFontRenderingFontSmoothingDisabled as many advices on the internet said. But nothing helped me and I declined an idea of using an external display.

But now I wonder what this setting actually does? I was playing with CGFontRenderingFontSmoothingDisabled on my MacBook Pro (15-inch, 2018) with Catalina and without external display and it seems to me that this setting does not change how the fonts are rendered on built-in dislay. No noticeable difference.

Can somebody please explain what setting CGFontRenderingFontSmoothingDisabled is supposed to do? Maybe I use it wrong or not able to see the difference.
CGFontRenderingFontSmoothingDisabled is used to enable/disable subpixel antialiasing. Apple disabled it automatically on macOS Mojave which may cause problems on some displays (especially non-LCD) but it improves the rendering on Retina screens. 

Here is an example :