Previewed at WWDC, launching in the fall.
Apple Releases Second Beta of macOS High Sierra 10.13.5 to Public Beta Testers
Beta testers who have signed up for Apple's beta testing program will be able to download the new macOS High Sierra beta through the Software Update mechanism in the Mac App Store.
Those who want to be a part of Apple's beta testing program can sign up to participate through the beta testing website, which gives users access to iOS, macOS, and tvOS betas.
macOS High Sierra 10.13.5 once again introduces support for Messages on iCloud, a feature that was present in macOS High Sierra 10.13.4 betas but pulled ahead of the release of the updated software.
Messages on iCloud is designed to store your iMessages in iCloud for improved syncing. Right now, incoming iMessages will be sent to all devices where you're signed into your Apple ID, but it's not true cloud-based syncing because your old messages don't show up on new devices nor does deleting a message remove it from all of your devices, both features enabled through Messages on iCloud.
The Messages on iCloud feature also allows your older iMessages to be stored in iCloud rather than on your iPhone, iPad or Mac, saving valuable storage space. Older attachments are also stored in iCloud.
No other major outward-facing changes were discovered in the first two developer betas, but the update likely includes bug fixes and improvements to address issues discovered since the release of macOS High Sierra 10.13.4. Because Apple does not provide detailed release notes for macOS High Sierra updates, we may not know exactly what's included until the new software is provided to the public.