Equivalent circuit (series or parallel)
Actual capacitors, inductors, and resistors are not ideal components of pure reactance and pure resistance. They usually have both resistive and reactive components. An actual impedance element can be simulated by an ideal resistor and an ideal reactor (inductance or capacitance) in series or in parallel.
It can be converted mathematically with a formula, but the two forms are different, and the inconsistency depends on the quality factor Q (or loss D).
For components with low impedance value Z (high value capacitors and low value inductors) use the series equivalent circuit. For components with large impedance Z (low value capacitors and high value inductors) use parallel equivalent circuits.
The equivalent circuit is determined according to the actual use of the component. For example, for a capacitor, a series equivalent circuit is used for power filtering, and a parallel equivalent circuit is used for an LC oscillator circuit.