A swing source is a utility supply or generator in which the real power flow is not given. In any power flow solution, the net real and reactive powers cannot be fixed in advance at every bus because the losses in the system are not known until the solution is complete. The swing or "slack" sources supply the difference between the given real and reactive power into the system at the other buses and the total system output plus the losses in the system.
Voltage magnitude and angle are given for each swing source. Real and reactive power flows from (or to) this source are determined as part of the solution.
In most large industrial plants, the utility supply is designated as the swing source. In-plant generation is usually designated as PV (voltage controlled), or PQG (fixed power and vars). If multiple utility ties exist in the plant, they are usually all designated as swing, unless you know the specific power and or voltage values at a specific tie point. For plants which are isolated from a utility supply, the largest generator is usually set as the swing source.
For commercial systems, the utility supply is almost always designated as the swing source for the system. Commercial systems rarely have sufficient generation to allow specification of a generator as a swing source. For emergency conditions, however, this is often done.
For utility systems, one or more large generators or an interchange point is usually designated as the swing source.