Currently Plex consider a media file to be played if the file is played to about the 90% point. This is OK for movies, TV and most other videos BUT it is totally inappropriate for longer media like Audiobooks.
There are many audiobooks that have a length of over 20 hours and not a few that are above 60. So, if you are playing a 20 hour audiobook and you get to the 18 hour mark and you need to stop for any reason when you go to resume it the start place will be back at the start of the book and not at the 18 hour mark.
This forces you to either never shutdown the playback or FF through the first 18 hours which takes a fair amount of time and to add to the insult many devices, like the Fire TV, do not FF or RW audio files at all correctly.
What I would like to see is a setting by library where you change the percentage that Plex considers as “complete” or for audio files an automatic move to the 100% point as it makes no sense for most audio files to not be able to be resumed before the end.