I have question about IR

[Garteal]

Auto will set the internal resolution to exactly 1920x1080 if that's the resolution you selected.
3x internal resolution is (1920x1584) a big bigger than 1080P and might look better than auto.

Personally I'd recommend using the multipliers as using auto (forced IR) might cause issues with certain games.
If you're going to use auto, make sure you're using the multiple of 640x528 option.