Sorry, but I dont speak engineer, can you explain in layman's terms? Please?
He means:
Unity is the point where no gain nor attenuation is added to the signal.
The core of the recorder is the codec chip with optionally it's own PGA (Programmable Gain Amplifier?).
When the PGA is unsused that amplifier is at unity.
The reference signal (which varies per chip) will then reach 0dbFS on the scale.
Therefor the level of the reference signal is important.
For the HR that level is 1Vrms when the WM codec is powered at 5V for the analog part.
When you insert a signal different from 1Vrms into the HR this means that either attenuation or amplification neds to be added, outside of the codec chip.
This is not what you mean with unity.