It goes something like this:
The RAAF expected the USAF to buy the KC-30A, so called their aircraft the KC-30B, but the USAF ordered the KC-45, so the RAAF changed the name of their aircraft to KC-30A. The RAF decided to call their aircraft the KC-30B and KC-30C. Then a change of personnel prompted the requirement for a name, so the aircraft was called the Voyager. The KC-30B and C designators were already in circulation, so the Voyager post-nominals became KC Mk 2 and 3. Then the USAF ditched the KC-45 for the KC-46. The Italians and Japanese now have KC767 International Tankers and the remaining Airbus customers have A330 MRTTs, which is what is written on the front of most of everybody's operating manuals; except for those that say Boeing of course.
Hope this helps.