The information online about M4A vs M4B is wrong. There is no difference other than the file extension. The Wikipedia article links to a lifewire.com article about the bookmark claim. This container format can store XMP metadata, and you can certainly have a player that saves a playback position in the file's XMP metadata, regardless of its M4A or M4B extension. But every player I know of doesn't do that. They store playback positions in their own internal database.
This claim seems to originate from the fact that the old iPods only remembered the last played position on M4B files. But that's entirely a player convention, not a file format convention.
This claim seems to originate from the fact that the old iPods only remembered the last played position on M4B files. But that's entirely a player convention, not a file format convention.