For Spectral Weighting, you can choose between A, B, C, or Flat.
A–Weighting is typically used because it has a frequency response curve that is most like the human ear at most volumes – dropping off in sensitivity as the frequencies get lower.
B and C are increasingly more linear than A — following how the human ear gets more linear at louder volumes. These are intended for increasingly loud sound pressure levels and checking low–end levels that are rolled off in the A–weighting.
Flat has no filtering and can check how the sound levels are at all frequencies as the microphone “hears” it.