CLZ Cloud is a free service for storing and backing up your collection database in the cloud. It is also the recommended way for syncing data between devices, e.g. from your Windows or Mac Book Collector to CLZ Books on iOS / Android, but also to sync between your mobile devices.
If you own multiple devices, e.g. iPad and iPhone, you will need a free CLZ Account so that you can sync your data through CLZ Cloud between your two devices.
Using the CLZ Cloud service is free for everyone, including Standard edition owners, Pro edition owners, and owners of the mobile apps.