Apple made an important change in Caching Server in Server 5. By default, it will now cache personal iCloud data in addition to the usual apps, books, and updates. While I guess there can be some value here in saving bandwidth, I seriously doubt most sysadmins would consider this worth the potentially massive storage requirements. On my home server, for example, I was already up to 60GB of iCloud data. And that is for two people!
There is no way to turn it off in the GUI, but an Apple Service Provider in the UK found a way to disable it via command line. Check out their post for the details.