Looking through the more than 2 GB contents of the macOS 10.13.5 update, this is another very substantial update to High Sierra which appears to go well beyond adding Messages in iCloud and the declared security fixes.
Substantial updates are made to Safari, bringing it to version 11.1.1, iBooks (version 1.15), Automator (2.8) and many actions, Mail (version 11.4, addressing S/MIME vulnerabilities), System Information (version 10.12), and the Finder (version 10.13.5).
Other apps which are updated include:
App Store, Calculator, Calendar, Chess, Contacts, DVD Player (which remains 32-bit, though), Dictionary, FaceTime, Font Book, Maps, Messages, Mission Control, Notes, Photo Booth, Photos, Preview, QuickTime Player, Reminders, Stickies, TextEdit.
Those updated in /Applications/Utilities include:
Activity Monitor, AirPort Utility, Audio MIDI Setup, Bluetooth File Exchange, Boot Camp Assistant, ColorSync Utility, Console, Digital Color Meter, Disk Utility, Grab, Grapher, Keychain Access and Certificate Assistant, Migration Assistant, Script Editor, Terminal, VoiceOver Utility.
Other significant updates include:
- /Library/WebServer has been extensively updated
- Photo Library Migration Utility
- most, if not all, kernel extensions have been replaced
- many public and private frameworks have been updated
- many tools in usr/bin and /usr/sbin have been updated
- eficheck, and its Allow List has been substantially updated to reflect EFI firmware updates.
APFS is something of a puzzle. Although the 10.13.5 update replaces its kernel extension and all its tools with more recently-built versions, the declared version number remains 748.51.0, which is unchanged from that in macOS 10.13.4. There is no evidence here that it has extended support to Fusion Drives, although it is now almost exactly a year since Apple announced at WWDC 2017 that APFS already supported them.
The bundled EFI firmware update should increment the reported Boot ROM version of almost every Mac, apart from those which have already undergone update during beta-testing of this update. I have updated my full listing of version numbers to take this into account.