The -I series allows on NVR FR processing of 4x FD cams. In other words applied Face recognition processing to cams that don’t natively support it by processing AI against the stream. In the regular NVR (no -I) AI is supported but only if supported by the cam. I.e If ANPR is supported by cam then supported by NVR, if Face Recognition supported by cam then supported by NVR. The regular NVR cannot apply AI processing on stream to turn an FD (Face Detection) stream into Face Recognition results.
HTH