The dissipation factor tan δ is the quotient of the active and reactive components of the impedance.
The losses occur mainly in the dielectric and are represented by R in the equivalent circuit diagram.
Parallel to R is the insulation resistance Ris, which, in fact, only affects tan δ at very low frequencies.
Further dissipation is caused by the finite conductivity of the electrodes and the transfer resistance between the electrodes and the terminating wires.
This is represented in the equivalent circuit diagram by the series resistance r. L represents the remaining self-inductance.