After a lot of digging around I've finally figured out how the sub- and super sampling options work:

• The subsample option scales down your resolution to 70% of your native resolution
• The supersample option scales your resolution up to 150%

Now there is a cutoff point where these stop working:

• The subsample resolution has a minimum width of 800 pixels, anything below will use native resolution instead.
• The supersample resolution cuts off at 2880 pixels (150% of 1920)

So what does this mean in bookah speech?

• Subsampling does not work on resolutions below 800 pixels width
• Supersampling has reduced effect on resolutions above 1920 pixels width
• Supersampling bugs out the bloom effect at all at resolutions above 2880 pixels width

Go to the Sub- and supersample resolution calculator