Brass pole pieces are non-ferrous and do not conduct the magnetic field to the strings. I have used a row of brass screws to eliminate the magnetic field from one coil in a humbucker to make it act more like a single coil pickup. I’ve used dummy poles (brass screws) in a stereo pickup to make the pickup look traditional but having one string go to the left channel and the next string go to the right channel etc. I’ve had pickups sent to me for repair that have had the pole pieces changed accidentally with brass screws that cut down the magnetic field to the strings which eliminates output and fullness, especially in humbucking pickups.