See the Working with Unknown Cameras and Photographs manual/help section for details on when and how this is used for dealing with photographs taken with unknown cameras. To use this feature you require control points (or Axes Constraints) to be marked on the photograph in question. If you turn these options on and have insufficient control points, the photograph will not be processed.
A small “+” or “-“ button to the left of the property heading is can be clicked to contract or expand the Inverse Camera settings. When expanded, three properties are shown: focal length (click the down arrow for this setting and choose TRUE when you want to solve for the focal length of the camera that took this photo), principal point (click the down arrow for this setting and choose TRUE when you want to solve for the x,y principal point of the camera that took this photo), and format aspect (click the down arrow for this setting and choose TRUE when you want to solve for the aspect ratio of the format size of the camera/scanner that took this photo).
If you are using Control Points to do Inverse Camera processing you must meet some minimum requirements. If you are solving just focal length in Inverse Camera (most common) then each photograph, that has InverseCamera focal length turned on, must have four or more Control Points with a good spread. If you have asked for principal point solution also you will need an additional two Control Points and for format size an additional one Control Point. So if you have all three Inverse Camera parameters turned on for solution you will need at least seven Control Points on that photo.
See the above mentioned section for information on when and when not to turn on the solution for various camera parameters. Note that if you get a solution it could be wrong or inaccurate if you solve for the wrong set of camera parameters.