I wouldn't call them theories ; rather, they are functions of the radio/receiver.
Squelch: a setting which silences the receiver until a signal of a given strength (or higher) is received. It is adjustable (usually, a knob) for varied conditions.
Attenuation: some receivers may be adversely affected by very strong signals. Setting attenuation on actually weakens the signal being received at the antenna, allowing it to be properly received. It does not silence the receiver.