The flat line is the setti

The flat line is the setting to disengage the roll-off circuitry. This means the output of the mic will be flat and unaltered.

The sloped line is to turn the roll-off circuitry on and will alter the sound by reducing the low frequencies.

I would suggest to leave it switched to the “flat” setting unless you specifically need the reduced bass response (I.E. for wind).

Jeff