macOS High Sierra
About required security updates
Your Mac checks daily for required security updates and notifies you when one is available in App Store. If the update does not require you to restart your Mac, it’s installed automatically after a specified amount of time. Otherwise, the update is installed the next time you restart your Mac.
If you prefer, you can always manually install required security updates.
Choose App Store > Preferences, then deselect “Install system data files and security updates.”