I just noticed today whilst playing from music I ripped onto my NAS that I am getting an “HD” icon on the screen. I looked at the Sonos blog and they mention HD music but then say there’s no accepted definition of what that means - sampling rate, bit depth etc. So I wonder what this is telling me or what criteria for showing the HD icon are?

This file is FLAC 44.1KHz (There doesn’t seem a straightforward way to get bit depth in Mac OS).

NB I tried another file I have and that is m4a and 44.1 KHz and also shows HD