[Elecraft] K4D Receiver Question

[Roy Morris, Jr.]

Considering the high noise level on the 75 meter band, has anyone had to use both 21 db of attenuation with PRE/ATTN button and additional attenuation with the RF/SQL encoder?  All illustrations have shown the RF/SQL encoder set to zero (default),  Is the RF/SQL encoder to be used only for squelch?  Under what conditions is it used for RF if not for attenuation?  Thanks,  Roy Morros