RMS is Root Mean Square. Aren’t you glad you asked?
RMS is a common measure of electrical energy (power company) that happens to work out to loudness when applied to audio. There’s no shortage of different ways to measure sound loudness, so even if RMS doesn’t work out exact, it’s really close and every engineer with a pulse knows what it is.
So the three technical measurements for Audiobooks are too loud overload, general loudness and quiet noise.
https://forum.audacityteam.org/t/measure-between-23db-and-18db-rms/32770/16
If you can pass audiobook standards, you can announce for just about anybody.
It’s not that easy.
Koz