As usual, Apple’s release notes for the Supplemental Update to High Sierra, released on 5 October 2017, fall short of describing what it actually updates.

Here’s a list of the significant changes:

• Mail.app is updated to version 11.0 (3445.1.7)
• Safari.app is updated to version 11.0 (13604.1.38.1.6)
• Disk Utility is updated to version 17.0 (1626)
• Keychain Access.app is updated to version 10.0 (55237.1.1)
• System Image Utility.app is updated to version 10.13 (820)
• Certificate Assistant.app is updated to version 5.0 (55174)
• Installer.app is updated to version 6.2.0 (920)
• Remote management is updated, with new versions of ARDAgent.app, its VNC Server and Screensharing bundles
• Setup Assistant.app is updated, together with several of its plugins, and migration
• APFS is updated throughout, to version 748.1.47
• Updated frameworks include AppKit, Cocoa, GameKit, SafariServices, and security frameworks
• The Startup Disk preference pane is updated
• Many private frameworks are updated, including several Swift ones
• AppleProRes Codec is updated
• Among the command tools updated are disktool, diskutil, and spctl.

There are also a lot of EFI firmware updates, which I will record, where necessary, in my separate article listing firmware versions, and many SMC updates too. It isn’t clear which of these may be duplicates of those in the original installer.

That sounds more like 900 MB of update, I think.

Supplemental Notes

See the comments below that reveal that only one of the apps listed above has a changed version from that in the original install. Of particular note is Disk Utility, which shipped in the first High Sierra install at version 17.0 (1626), and again in the Supplemental Update at that same version. However, checksums on the app binary itself confirm that the two versions are different, and they have different timestamps too.

The bug in Disk Utility is actually reported to have been in StorageKit, one of the private libraries. The original version of StorageKit shipped was 1.0 (388), and that in the Supplemental Update is 1.0 (388.2).

The APFS version number is that in both /System/Library/Filesystems/apfs.fs and /System/Library/Extensions/apfs.kext. The last version in Sierra 10.12.6 is given as 0.3, but in the later versioning should be considered to be 249.7.6. That in the first ship of High Sierra is 748.1.46, while that in the Supplemental Update is 748.1.47. That gives you an idea of the wide gulf between the version in Sierra and that in the initial version of High Sierra.

Thanks to Miles Wolbe for his help in this additional information.