Apple today pushed out a second Rapid Security Response (RSR) update for iOS 16.4 / macOS 13.3, which is the fourth RSR update since Apple introduced the Rapid Security Response update mechanism.
Apple intends to use Rapid Security Response updates in the future to quickly address security vulnerabilities that have been identified and are being actively exploited. This way Apple can not need to wait for regular updates and then push them to users, but can quickly fix for certain vulnerabilities.
This update appears in the same location as the normal update and can be found in the Settings app. This update appears as iOS Security Response 16.4 (b) and macOS Security Response 13.3 (b).
Users who trigger the Security Response update can revert to a previous iOS or macOS version during this test. Apple tested this feature in a previous Beta release.