This is the camera tilt down from the horizontal measured in degrees. The default value is 45o.
Tilts shallower than 30o are not recommended as the keystone distortion becomes extreme and small changes in camera angle or focal length will result in very large differences in the size and shape of the group. This also means that minor errors in measuring and setting up the group on site will be more noticeable.
You can change the default camera tilt in Settings (Pro version only).