The clearest explanation is in one of the updates on that page:
"The most obvious problems arose with iOS users who transferred files from Windows (which prefers a different normalisation form to HFS+) which were named using Korean and other character sets, although this even included European languages with accented characters like ñ and é. There’s a chilling series of messages on the Apple Developer Forums in which an iOS app developer details how users running iOS 10.3 were transferring files using iTunes for Windows, but could not access those files once they were on an iOS device."
The "chilling series of messages" takes you to a link which details how File sharing through iTunes on Windows specifically was affected and that a similar file transfer product on Windows iMazing was able to fix to fix it by simply normalizing before transferring.
( The "bag of bytes" response from Apple asks developers to do exactly that btw )
That begs the question that why iTunes itself ( on Windows ) wasn't able to do the same thing as iMazing ?
Anyway that's moot now - As mentioned earlier in this thread iOS 10.3.3 and iOS 11 are changing behaviour w.r.t. this.
https://mjtsai.com/blog/2017/03/24/apfss-bag-of-bytes-filena...
The clearest explanation is in one of the updates on that page:
"The most obvious problems arose with iOS users who transferred files from Windows (which prefers a different normalisation form to HFS+) which were named using Korean and other character sets, although this even included European languages with accented characters like ñ and é. There’s a chilling series of messages on the Apple Developer Forums in which an iOS app developer details how users running iOS 10.3 were transferring files using iTunes for Windows, but could not access those files once they were on an iOS device."