All of the above
That number is also the same used for {id}
That ties the DB entry to a specific Book Folder. You may have intentional duplicates. eg You have both English and German editions. only 'Languages' (ISBN is often wrongly entered)in the Metadata make this obvious
Do read the FAQ at the main Calibre Help page
DO NOT mess with the file/folder names. That breaks the linkages stored inside
My Kindle Content (folder)
. That seriously messes with the last rule. Calibre needs an Exclusive set of folder (AKA 'The Library').
The only Adds. Deletes or changes need to be done by Calibre so everything is properly recorded in the DB.