According to Wikipedia: "The Airbus CC-150 Polaris is the designation for the civilian Airbus A310-300s which have been converted for use as the primary long distance transport aircraft for the Royal Canadian Air Force.
...Two of the five CC-150s have been converted to air-to-air refueling tankers for the CF-18 fleet as CC-150Ts. This was a capability that was lost when the CC-137s were retired. The conversion is part of the Multi Role Tanker Transport (MRTT) program. The MRTT program was initiated because of a German Air Force requirement and provided a cost effective solution for the Canadian Forces."