The extraction of parasitic circuit models is essential for various aspects of physical verification such as timing, signal integrity, substrate coupling, and power grid analysis.
As circuit speeds and densities have increased, the need has grown to account accurately for parasitic effects for more extensive and more complicated interconnect structures.
That form may be a simple analytic parallel plate capacitance equation or may involve a full numerical solution for a complex 3D geometry with wave propagation.
The first uses a differential form of the governing equations and requires the discretization (meshing) of the entire domain in which the electromagnetic fields reside.
Those sources can be physical quantities, such as the surface charge density for the capacitance problem, or mathematical abstractions resulting from applying Green's theorem.