The system settings allow you to change the operation levels of the inputs/outputs from line level to instrument level. Here is an explanation of line level vs instrument level signals: https://www.gearank.com/line-level-vs-instrument-level/
There’s not too much to worry about here, the H90 has a lot of headroom and most pedals will work fine with either level settings. It is most important if you are sending a (hot) line level signal to the H90 and you see input LEDs are clipping, then you need to adjust the input to line level or reduce the output of the instrument.
