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."
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."