Is the horizontal width that the Previzion camera sees at any unique combination of zoom and focus. The viewable area value is defined by the combined effects of entry pupil and focal length.
The Previzion Camera
In order to understand how this impacts Previzion rendering, study the illustration below. It shows the difference in positions of the physical camera and the virtual camera. The virtual camera is the position that Previzion renders from. The virtual camera position is defined physical camera position with the entry pupil offset added in.
Calculating Viewable Area
The illustration below gives a more mathematical approach to understanding viewable area and shows how entry pupil and focal length both are important factors in describing what the virtual camera (aka previzion, aka your-post-renderable camera) sees during render time.