The KMD-540 is the actual MFD unit included in the KMD-550/850 systems. Therefore, KMD-540 + Install Kit & Database = KMD-550. And, KMD-540 + KAC-501 radar module + Install Kit & Database = KMD-850. The KMD-550 and KMD-850 are merely system reference designations. The actual MFD unit does not have the KMD-550/850 part on it.