Viewfinder
Preview what the Camera sees when attached to a PCam.
It's accessible from the bottom panel in the editor labelled Phantom Camera.
The viewfinder rendering of the scene will only work when the combination of a Camera, PCamHost and PCam are present and set up correctly.
Video Example
Dead Zones
When Follow Mode is set to Framed, dead zones will also be visible in the Viewfinder and, if enabled, when playing the game from the editor.
Note
Dead Zones will never be visible in build exports.
Multiple Phantom Camera Hosts

The moment two or more PCamHost are in a given scene, a small button will appear in the bottom left of the Viewfinder editor panel. Pressing on the icon, will open a list that displays all the PCamHost instances in the scene. Each list item contains two buttons; a small select icon, which will select the given PCamHost from the Scene Hierarchy, and a bigger button that will swap the Viewfinder to display the output of that Camera2D/3D that PCamHost belongs to.