What is the difference between Primary Magnification and System Magnification?

Primary magnification (PMAG) is the magnification of the lens alone. It is the ratio between the camera's sensor size and the field of view; put simply, it is the "work" done by the lens. System magnification is the total magnification from the object to the image on the monitor; this is the "work" done by the entire system. It is the product of the PMAG and the camera-to-monitor magnification (the ratio between the monitor size and the sensor size). Edmund Optics typically uses horizontal values for the sensor-to-field of view ratio and diagonal values for the monitor-to-sensor ratio.