Client web server is responsible for creating pass packages for download onto user’s devices. A pass package consists of a JSON file that defines several data elements, including any bar codes, images, and optionally localization files (language-specific images and strings). Each pass has a type, which needs to be registered with Apple like bundle ids are, e.g. pass.com.example.event-ticket. Each pass needs to have a unique serial number, which is an opaque string used to identify the instance of a pass. To update an existing pass, a new pass needs to be created with the same serial number. Passes are automatically synced between a user's devices, and to their Apple Watch.