Direct Stream Digital Audio

What is high resolution audio and how does it differ from Mastered for iTunes?

What is high resolution audio and how does it differ from Mastered for iTunes?

With Amazon, Apple, Qobuz and others beginning to offer streaming services for high resolution audio, we are about to see the "war of words" and definitions hit music lovers hard.  Brace yourself as the mainstream enters the audiophile world and subscribes to Amazon's Ultra HD Music service for high definition audio.  What does it all mean?
The good news is, we may have more high resolution files to choose from. The consumers Have been way ahead of the professional audio community in this regard.  The labels will be struggling to find music created in 192WAV since most recording engineers poo-pooed the notion of high resolution for a long time.  
I wish I knew the answer, but since our focus is DSD, the good news is ALL DSD is high resolution, high definition, high end... whatever you want to call it.  
But, as we like to say, the no matter what format you listen to, if it was recorded badly in ANY format, it's just not a good sound.
Here's the document that NARAS sent out in 2017 christening 48/24bit as the minimum requirement for high resolution.  (Note, not 44.1 / 24bit).

Where it gets ugly with Apple, is that they describe 44.1/24 bit as "high resolution".  Note that all of their files, whether 48, 96 or 192, get 'downsampled' to 44.1/32bit float.  Then processed again to be made into a lossy file called AAC (equivalent to mp3) in variable 256kbps.  AAC Lossy is 'lossy' and is shouldn't be considered high definition, though it might sound better than many mp3's.
Qobuz use of High Res logo is anything 24bit.
Bottom line, to be safe, get 192/24bit files WAV or FLAC

24 bits
44.1 kHz - Stereo

Condensed NARAS white paper - excerpt
 By the simplest definition, hi-res audio is lossless audio that is better than CD-quality in both sample rate and bit-depth. Since the Red Book specification for CD-quality audio requires a 44.1 kHz sample rate, 16-bit PCM format, the lowest possible format to be considered hi-res audio is 48 kHz/20-bit PCM. However, even though 48 kHz/20-bit audio quality is technically hi-res audio, the recommended minimum resolution for a recording project is 48 kHz/24-bit PCM. At 48/24, all modern computers are capable of recording and playing back large sessions with high track counts, along with a substantial number of plug-ins. The conclusion drawn by this committee is that tracking, mixing, and mastering at a resolution of 96 kHz/24-bit or 96 kHz/32- bit float PCM is preferred, providing true hi-res audio while imposing less burden on the computer CPU and allowing for higher track counts than when recording at 192/24, 192/32-bit float, or higher. There are two common digital audio formats for recording, mixing, and mastering hi-res audio: Pulse Code Modulation (PCM) and Direct Stream Digital (DSD). 
Recommended Formats Minimum Quality / Preferred Quality 
Minimum = PCM 48 kHz/24-bit 
Recommended = 96 kHz/24-bit or better 
Minimum DSD 2.8224 
Recommended MHz 5.6448 MHz or better  
Condensed MFiT description..Mastered for iTunes is now called Apple Digital Masters
In order to be Apple certified the client must provide a high resolution master to us for the mastering session. The master must be at least 24-bits, with a sample rate of at least 44.1K, though higher is recommended if possible. If a 16-bit master such as an audio CD, upload through our MUA, or 16-bit WAV files is submitted, we cannot complete the Apple Digital Mastering process.