A quick, plain-terms explanation of what's actually inside a JW Library backup.
A .jwlibrary file is the backup format created by the JW Library app when you use its Backup and Restore feature. It contains your personal study data — notes, highlights, bookmarks, tags, and playlists — everything you've added on top of the publications themselves, not the publications.
Under the hood, it's a ZIP archive with two things inside: a small manifest.json file describing the backup (when it was made, which device, a checksum used to verify it wasn't corrupted), and a userData.db file — a SQLite database holding your actual notes, highlights, bookmarks, and everything else, in a specific set of tables JW Library expects.
That specificity is why you can't just rename any ZIP file to .jwlibrary and have JW Library accept it, and why tools that build a JW Library backup from scratch (rather than starting from a real one) sometimes produce files that get rejected on restore — see our guide on fixing a failed restore if that's what brought you here.
If you're trying to combine the data from more than one .jwlibrary file into one, that's exactly what JW Fusion does — see the guide on merging backups from two phones.