The term Scope mode refers to measuring only the input signal (e.g. measuring with an oscilloscope).
The frequency response of a system (e.g. a loudspeaker or an amplifer) is found by measuring the output of the system from a measured or known input signal. The gain of the system will NOT change if we increase the signal we feed the system with (e.g. if we have an amplifier with gain 30 dB, the gain will still be 30 dB if we increase the level to the amplifier input). In this case we are not measuring in scope mode since we want to determine the system gain.
But if we want to find the output level of a loudspeaker in SPL, the SPL obviously dependens on the gain of the signal we feed the loudspeaker with. Therefore we have to plot in scope mode, since scope mode only considers the measured signal on the microphone and does not compensate for the signal level we feed the loudspeaker with.