A higher value will result in excessive smoothing and result in a less clear audio. Use ‘Frequency Smoothing (bands)’ to widen the noise reduction to neighbouring frequency bands.
Like NR, settle with the lowest value that lessens noise without harming the part you want to hear. A higher ‘Sensitivity’ value will remove more noise but could also remove essential bits. Use the lowest value that reduces the noise sufficiently without damaging the rest. Increasing Noise Reduction (dB) value will lessen the noise but may also remove essential parts of the audio track. In case the effect is removing more than the noise then finetune the settings.
Audacity will use that portion you just selected as the noise signature.
