APS stores information as XML documents. Every document is stored with a version number. Whenever you save changes on a document, APS will save the document under a new version number. The previous version of the document will not be overwritten. You can always review and restore previous document versions.
This creates a version history which makes it safe and comfortable to apply changes to your collection. For example, if you find out that removing the author from a book wasn’t your brightest idea, you can easily revert the document to its previous version and discard your changes. Even deleting and undeleting entire documents is possible.