In the early days of my iTunes usage, I became terrified by the word Sync, because I learned the hard way that iTunes could modify the very structure and content of the folder on my PC containing actual digitized music files. Perhaps we need a new language with words like "downsync", "upsync" and "crosssync" to at least describe what's happening when we push a button merely labeled "Sync".
The debate on whether MusicBee should upsync anything from a target device, like play data from an iPhone, is a separate issue. For me, I don't want anything I do on an iPhone or my SONOS system to mess with how I have structured my actual digitized music files on my PC.
In fact, what does the word "library" mean? In both iTunes and MusicBee, a library is just a collection pointers to the "real library" that contains digitized music files, sort of like a card catalog in a traditional book library that points to where actual books are in the stacks. So "pointerlibrary" and "stacklibrary" come to mind as useful distinctions.
The problem with trying to upsync anything from iTunes, in particular, is that it's a moving target. Apple cavalierly makes changes to fundamental file structures used in iTunes as they see fit, with little regard for the consequences to downstream software. The SONOS company, and its user community, has been struggling with this issue for years. In fact, it's the very reason I've come to use MusicBee instead of iTunes to curate my stacklibrary. I'd get things set up and working just fine, and bingo, a new release of iTunes would break it. If MusicBee does anything in the future to upsync data, I hope it's clearly labeled, and, by default, off.
iTunes refugee. Six SONOS zones. Harmony Elite Remote/Hub. Alexa enabled house. Using MusicBee for curating music (managing library, creating playlists, etc.). Thank God I'm a Systems Engineer.