Render scale is the percentage of your resolution. So 100 is always the same resolution as what you have set in the graphics settings. Over 100 is super sampling, which doesn’t have a big impact if you already have anti-aliasing enabled like TAA. The image will be down scales afterwards to match your set resolution. If you set it below 100, the GPU will render the frame in that lower resolution, and afterwards the image will be upscaled to match your set resolution.
If you open developer mode’s FPS meter you can see all resolutions used, so if they are all the same it uses 100.