The MaxAxisFieldOfView property sets how many degrees along the longest viewport axis the camera can view.

When the longest axis is the vertical axis, this property will behave similar to the Camera/FieldOfView|FieldOfView property. This is generally the case when a device is in a portrait orientation.
In a landscape orientation the longest axis will be the horizontal axis. In this case, the property describes the horizontal FOV of the Camera.

See Camera/FieldOfView|FieldOfView for a more general explanation of FOV.