Originally Posted by kenr276
Just to let you know that when you use the exclusive shelves as tags the tags themselves are not exclusive. I now have books that include both read and to-read
If I understand you correctly, then yes, it is the case that applying tags to books based on the shelves they are on in Goodreads is an additive process, it never removes tags from a book. Perhaps on a genre specific column you could get away with replacing all values with the "current" one from the shelf. But it would be annoying for users with their main tags field to lose any other tags they had applied.
The workflow I had in mind to handle the scenario you described was to not actually use the shelves as tags feature for that purpose. Instead the "sync from shelves" feature was really designed to ensure your Calibre tags were kept in sync with a shelf for where the tags are expecting to be changed. So if you want to use tags in Calibre to represent "ToRead" versus "Read", then you would setup sync action rule for your "read" shelf in Goodreads to "Add tag of Read" and "Remove tag of ToRead".
Using shelves as tags is really targeted at shelves where you will not be changing the values of them, like a genre. And in the hopefully low likelihood where you do change the value, it is your responsibility to remove any old tagged value from Calibre so it will only have your new tag.