This will add the resolution corresponding to your monitor resolution and mode setting to the in-game resolution list if your monitor's resolution is supported by FSR.
Example:
If you choose 'ultra' and your monitor is 1440p (2k), it will add this entry to the in-game screen resolution list:
1970x1108
* You must then choose that resolution in-game and FSR will scale it up to your monitor's 1440p size.
If WINE_FULLSCREEN_FSR is enabled but no mode is set, the 'performance' option for screen resolution (if supported by FSR) will be added.
Again, the full list of resolutions can be found below for reference: