Apple’s release notes for OS X Yosemite Developer Preview 4 reveal that the company will be wiping CloudKit data tomorrow.
This data has to do with the new iCloud storage APIs in iOS 8 and Yosemite as well as with the upcoming iCloud Drive online storage feature. It’s best to safely store anything of importance before tomorrow’s wipe:
CloudKit Note: All public CloudKit databases are scheduled to be emptied on Tuesday, July 22nd.
Update: Apple has emailed developers about the wipe, noting that iCloud Drive, Photos, and other iCloud-related products besides CloudKit storage will not be wiped: