SUMMARYWhen complex structures are divided into subdomains for electromagnetic "eld computations, it is necessary to specify which of the tangential electromagnetic "eld components at the boundaries may be regarded as independent and which have to be treated as dependent. In this third of our three-part study, we establish some rules for the choice of primary and secondary "elds at the connection interface, i.e. at the interface between adjacent subdomains. The discretized "eld continuity equations at the connection interface provide the connection network whose canonical forms are illustrated, and it is shown that the normalized scattering matrix is symmetric, orthogonal and unitary.Tellegen's theorem is introduced in order to provide the basis for consistent choice of primary and secondary "elds and for deriving canonical forms of the connection network. The "eld problem is systematically treated by partitioning and by specifying canonical Foster representations for the subcircuits. Connection between di!erent subdomains is obtained by selecting the appropriate independent "eld quantities via Tellegen's theorem. For each subdomain, as well as for the entire circuit, an equivalent circuit extraction procedure is feasible, either in closed form for subdomains amenable to analytical treatment or via the relevant pole structure description when a numerical solutions is available. Moreover, circuit-based resonant expansions in the complex frequency plane are provided here for the analytic dyadic Green's functions in [1].