With the CC cloud storage, you decide manually what's uploaded. You have 20 gigs and those files stay in the cloud and are also synced to every computer. You can also access the files via a browser. The files stay intact and it may be used as a backup, but limited to the 20 gigs.
If you are asking about Lightroom mobile sync, Lightroom desktop creates smart previews, which are synced to the cloud and from there to Lightroom Mobile. It's not a proper backup, because the maximum resolution is 2560 on the longest side and it's heavily compressed. Smart previews of the photos in all synced catalogs are stored in the cloud and connected to your account, but they do not count against your storage quota. You can only sync files from one catalog at a time.
