Becuase UHF antennas are so variable in design (and hence, input impedance), this two layer material is taylored to the application, which is defined by the chipset in question. The technology employs "impedance tuning", which matches the electromagnetic parameters (real & imaginary - permittivity & permeability) constrained by the particular environmental factor that required the adjustment - generally metal.
The single layer material is universal, and works to isolate virtually any HF inlay from a metal surface. Available in a wide range of thicknesses, HFH-30 (@0.030") being the most popular.