m4a is for iTunes. I believe someone improved it to m4b (chapters) and now Apple products can't play it ...
That "someone" was Apple.
m4a is just music, for single tracks.
m4b is the original format for iTunes' audiobooks and podcasts; it's just m4a with a chapter list and bookmark tags added.
Technically, they're all mp4 files, the different extensions just indicate what is in them.
From iTunes it will also have DRM.
As we now know, Audible AAX is also M4B, but with a different encryption (that inAudible removes).
Since both Apple and Audible just want you to buy their files to play on their apps/devices, they have not wanted to publicly document the formats they use, so we have to go to 3rd parties for the details.
An Audiobook / podcast extension with AAC or ALAC encoded audio in an MPEG-4 container. Both M4A and M4B formats can contain metadata including chapter markers, images, and hyperlinks, but M4B allows "bookmarks" (remembering the last listening spot), whereas M4A does not.
And Apple products certainly can play them. You just have to know how to get them into iTunes.
Tested this on an iMac:
I have a m4b on a USB drive.
Plug drive into Mac.
On Mac, open iTunes (current version 12.8).
File menu: Add to Library -- find the m4b and select -- Click "open" at bottom of file screen.
Back to iTunes Library.
Near the top is a menu with different media. Probably set on "Music".
Click the up/down arrow next to choose from:
- Music
- Films
- TV Programmes
- Podcasts
- Audiobooks
Choose audiobooks, you should see your m4b, ready to play, or copy to iPod, iPhone.
If the file is already on your Mac, select it, ctrl-click to get the menu.
First option should be "Play".
Choose 2nd option "Open With" -- and then choose iTunes app. That should load it in the library as above, and start playing it.
Similar if you're using Windows to load on to iOS devices:
A file with the M4B file extension is a MPEG-4 Audio Book file. They're often seen used by iTunes to store audio books.
iTunes is the primary program used to both play M4B files on a computer as well as to transfer audiobooks to an iPhone or another iOS device. You can do this by adding the audiobooks to iTunes and then syncing your device with iTunes.
Start by transferring the M4B file to iTunes. In Windows, use the File menu to choose either Add File to Library... or Add Folder to Library... to browse for the M4B file. If you're on a Mac, go to File > Add to Library....
With the device plugged in, click or tap the phone icon in iTunes to switch the window to the iOS device. Choose the Audiobooks menu on the left side of iTunes. Put a checkmark next to ​Sync Audiobooks, and then pick whether you want to sync all the audiobooks from your iTunes library or just certain ones.
Now you can sync your device with iTunes to send the M4B file to your iPhone, iPad, or iPod touch.
