Apple seeded macOS 10.13.5 developer beta 3 on Tuesday, in keeping with iOS 11.4 developer beat 3. The biggest feature for end-users is continued testing of Messages in iCloud.
Messages on iCloud
This feature shifts local storage of iMessages to iCloud. Rather than storying everything locally and syncing everything up when you use your individual Apple devices, those devices will instead pull just recent messages from iCloud.
If you have lots of iMessages chats and group chats going—and if those chats are image, movie, or other files-heavy, this will save you gobs of space. For instance, my Messages folder is currently 11GB and growing. It will also make syncing on devices you don’t use every day a lot faster.