ADC RMS Noise (11nv@4.7Hz)
The RMS value of a noise signal, just like with any other type of signal, is the value that you would use to calculate the power of the signal, which is the parameter that designers are most interested in. After all, signal-to-noise ratios are power ratios, so you want to compare the RMS value of the signal to the RMS value of the noise, in whatever units are appropriate to the application at hand.

The "11 nV RMS @ 4.7 Hz" you see on the front page of the datasheet is just one entry (the "best" entry) from the table found on page 14, which relates different sample rates and gain settings to the overall noise figure.