https://arstechnica.com/gadgets/2023/05/seven-months-in-ios-and-macos-get-their-first-rapid-security-updates/

When it announced iOS 16, iPadOS 16, and macOS Ventura at its Worldwide Developers Conference last summer, one of the features Apple introduced was something called "Rapid Security Response." The feature is meant to enable quicker and more frequent security patches for Apple's newest operating systems, especially for WebKit-related flaws that affect Safari and other apps that use Apple's built-in browser engine.

Nearly a year after that WWDC and more than seven months after releasing iOS 16 in September, Apple has finally issued a Rapid Security Response update. Available for iOS and iPadOS devices running version 16.4.1 or Macs running version 13.3.1, the update adds an (a) to your OS version to denote that it's been installed.