The Power Envelope Plot shows the power envelope of all multiplexed active carriers. A red box shows where ALC is active, and these symbols are used for the ALC reference.
By default, the ALC reference is set to the highest-power symbol(s) in the waveform, and the amplitude of those symbols is shown as 0 dB in the Power Envelope Plot. You can choose a different portion of the waveform to be used as the ALC reference by clicking on that portion of the plot. The red box will move to the new portion, and the power level there will now be considered to be 0 dB. The ALC Reference Offset [dB] will be updated to show the difference between the power levels of the new ALC reference symbols versus the highest-power symbols in the waveform.