The "Direct to Camera Target" option removes the final blit Unity does when outputting to the screen, instead letting the post-processing stack output directly to that screen for a small performance benefit. However, this means that any cameras which render after this camera will not be combined with what has been rendered so far, effectively clearing the screen for those cameras. So, this option should only be used on the last camera that you are going to render in a frame.

Note that some users have run into this behaviour when upgrading from older versions to Unity 2019.1 or later. This is because the option was unintentionally enabled by default, but did not do anything in versions prior to Unity 2019.1. (The default has been corrected, as of Postprocessing 2.1.7). Disabling the option will restore the Unity 2018 behaviour.

The behaviour can also be controlled at runtime, via the API on the PostProcessLayer component; for more information, see https://docs.unity3d.com/Packages/com.unity.postprocessing@2.3/api/UnityEngine.Rendering.PostProcessing.PostProcessLayer.html