The domain Ω in which we seek the solution to the Poisson equation, consists of n subdomains Ωi, where the union of the subdomains is
Ω=Ω1∪Ω2∪⋯∪Ωn
and the intersection of two subdomains is always
Ωi∩Ωk=∅,i≠k.
Across the individual Ωi the physical parameters, e.g., conductivity, density, are usually constant.