I am not a heating guy.
He might be talking about an outdoor reset temperature control. This is used by the system to raise or lower the temperature of the circulating heat transfer material (e.g. water). It improves the efficiency of the system. But you have a heatpump that does airconditioning so I am assuming that the heating is also forced air.
The only fluid that could be controlled is the one internal to the heat pump. You can't control the temp, just the volume of flow. If you have a continuous variable speed compressor, or maybe a mnultispeed, there mught be something useful to do with outdoor reset. If you can find the manual for the unit it might explain this feature (if it exists).