This rule provides reasonable initial settings for the receiver Gain Modes. However, the optimum settings for a specific site may vary depending on the exact distribution of large amplitude signals and the desired operational requirements of the Spectrum Processor. If objectionable IMD spurs are observed in a particular preselector band, change the Gain Mode from Rural to Urban, or Urban to Congested. If greater small signal sensitivity is desired and IMD spurs are not present (or are sufficiently small to be ignored), change the Gain Mode from Congested to Urban, or from Urban to Rural. Rural provides the best sensitivity, but is more susceptible to IMD, while Congested provides the best immunity from IMD spurs, but sacrifices some sensitivity. Urban is a middle setting that provides good IMD immunity with minimal loss of sensitivity.