I copied and pasted from the Amazon web site. Some talk on the Amazon HD post about a higher quality than CD. Here is what Amazon says:
What audio quality does Amazon Music HD support?
Amazon Music HD offers lossless audio in two quality ranges: HD and Ultra HD.
HD tracks are 16-bit audio, with a minimum sample rate of 44.1 kHz (16/44.1 is also referred to as CD-quality), and an average bitrate of 850 kbps. Ultra HD tracks have a bit depth of 24 bits, with sample rates ranging from 44.1 kHz up to 192 kHz, and an average bitrate of 3730 kbps.
In comparison, most standard streaming services currently offer Standard Definition (SD) with a bitrate up to 320 kbps. These audio files use lossy compression, where details of the original audio are removed in order to reduce the file size. By contrast, Amazon Music HD preserves the original recording information to deliver the highest quality sound available, more than 2x the bitrate in HD and more than 10x the bitrate at the highest Ultra HD bitrate. Amazon Music HD will always play the highest quality content available, based on network, device capability and your selected settings.
So what is interesting is I tried to see what the cost of Ultra is, and I am not finding it anywhere. So its probably in the works. Time will tell. For me, Amazon defining HD as better than MP3 and then calling that Hi Rez is a bit disingenuous. It does nothing for me, but it will keep TIDAL and Qobuz on their toes. I prefer MQA with Roon/TIDAL and a full decode with a Meridain Ultra Dac. Best music I have ever heard. Been doing this for 40 years.