What resolution to set in-game when using fsr

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I feel stupid even asking this, but I haven't been able to find an unambiguous answer yet.

(I have a 4k monitor). What is supposed to happen when I set the game to 1440p and turn on Fsr Quality? Does it use the 1440p as input and output 4k OR does it use a lower resolution to output the 1440p?

As it is, I can't really tell the difference, but FSR seems to be doing little for my framerates.
 
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