I agree it’s all a bit confusing.
To put it all together
DSR enables the use of a higher screen resolution.
DSR multiplies the amount of pixels rendered
DSR 1.777x 1080p is 2560x1440
DSR 2x 1080p is 2715x1527
DSR 4x 1080p 3840x2160
With DSR on you should be able to select that higher resolution under full screen resolution in the graphics settings in FS2020. If you don’t then DSR does nothing.
SecondaryScaling does the same as DSR yet in game (and using in game down sampling)
SecondaryScaling multiplies the horizontal/vertical full screen resolution
SecondaryScaling 1.3333 x 1080p is 2560x1440
SecondaryScaling 2.0 x 1080p is 3840x2160
SecondaryScaling leaves down sampling until after everything is rendered and post processed.
PrimaryScaling is the same as RenderScale in the graphics options.
PrimaryScaling 1.0 is RenderScale 100.
PrimaryScaling multiplies the horizontal/vertical resolution after SecondaryScaling,
PrimaryScaling x SecondaryScaling x Full Screen Resolution is the render resolution.
PrimaryScaling 2.0 x SecondaryScaling 1.333 x 1080p would be 5120x2880 render resolution, with 2560x1440p post process resolution.
PrimaryScaling 0.666 x SecondaryScaling 2.0 x 1080p would be 2560x1440 render resolution, with 3840x2160p post process resolution.
The render resolution is what the environment gets rendered at, polygons, textures etc.
The post process resolution is where sharpen, color grading and TAA get resolved.
DSR allows you to start with a higher screen resolution, gives you more control over down sampling and gets better results when you use NVidea filters.