generator impedance and resistance
The resistive component of a generator output impedance comes largely from the physical resistance of the windings.  How you model this is up to you.

Usually we deal with systems intended to have a fixed voltage.  In that case, it is useful to model the generator as a voltage source with the inductance and the DC resistance in series.  That's the Thevenin equivalent.

The Norton equivalent is just as valid, but generally less useful or intuitive to use with the types of systems generators usually drive.