reflectarrays [60] Reconfigurable Intelligent Surfaces [1] Intelligent walls [57] Software-controlled metasurfaces [61] Smart skins [62] M. D. Migliore Clearly, the potential in terms of information transmission present in the DPL is limited only by the physical laws, which are inviolable. Consequently, the limits imposed at the level of the DPL are absolute. In this view, as already noted the DPL is a more fundamental level than the PL. The results reported in this chapter represent a further contribution in the long path towards a physical approach to communication, started soon after the publication of Claude Shannon's fundamental papers on information theory [10, 11]. This effort has shown interesting connections between information theory and a large number of different fields of physics. In particular, in 1969 Toraldo di Francia introduced the concept of Degrees of Freedom of an optical image [12]. Starting with this article, the optical community has actively studied the connections between optical images and information [12-18]. The work of Toraldo di Francia on optical degrees of freedom was also the starting point for the introduction of the Number of Degrees of Freedom (NDF) in electromagnetism in 1989 [20]. The NDF of the electromagnetic field was introduced in the framework of approximation theory as the minimum number of orthogonal basis functions required to represent the electromagnetic field within a desired error using a linear representation. The paper [20] belongs to a large set of papers devoted to the electromagnetic field approximation theory that includes also efficient bandlimited approximation of the field [21, 22], with a large number of applications in the framework of antenna measurement [23, 24]. The introduction of space multiplexing triggered extensive research on the connections between information theory and electromagnetism in both the com- munication and the electromagnetic communities [25-30]. The approach followed in a first phase was based on spherical expansion and was cons