The camera contribution value is also called camera mask. The calculation of a 3D data location requires two or more observers (cameras or sensors). When more than two observers contribute to the calculation of a 3D location, it is useful to record which of the observers contributed to the calculated measurement. The C3D point record allows up to seven observers (generally, but not necessarily, cameras) to record whether or not their data was used to generate the 3D Point measurement.
This is information specific to each data collection environment and can be very useful for debugging and quality control as it allows a user to identify the cameras (or observers) that produced information used by the 3D calculations that generate the 3D locations stored in the C3D file.