To simplify the situations are reduced to bivalent systems, with a source more virtuous (geothermal/hydrothermal) and a aerotermica less virtuous. In general, however, this does not is a  restrictive pattern, because of done it can insert several source systems who work together. The control acts considering a hysteresis on the source side to determine the limit below which triggered and the source more virtuous.

Since the virtuous source is, in this case, a source for the definition "benefit not guaranteed", the exchange capacity tends to decrease until it meets a too low value to enable its operation. Typically this value is a few degrees above the water freezing with not glycol. In special cases, such as where it is expected glycol (use of solar collectors) point SWAP can be lowered.

The source less virtuous but more available should be have an exchange capacity equal to the heat pump.
The most convenient source can have any capacity keeping in mind that if it is higher it may be convenient to think of a reservoir. If it is much lower than the SWAP it will still be limited by the control.

Heat recovery from industrial waste
Water with flow rates not guaranteed
Solar collectors
Polyvalent systems