Originally Posted by Petri Kaipiainen
There is no other measure or unit of this quality than dynamic range. It all boils down to dynamic range: ratio between the total maximum amplitude and the finest details in that waveform.
Generally we have finest details in 24 bit vaweform and lover quantisation noise. So it still make sense to use 18, 20 or 24 bit if we record to -6dB and normalize in postproduction. Because recording to -6dB we use only a half of the maximum amplitude and we add less quantisation noise to the noise of analog side.