Impedance boundary is generally considered as an approximate model for material interfaces. Considering an electromagnetic field in a certain wave-guiding anisotropic material it is shown that a slab of such a material backed by a perfect electric conductor (PEC) plane can be exactly represented by impedance-boundary conditions. As a result of the analysis, a novel explicit relation is derived between the surface admittance dyadic of the impedance boundary and the material and geometric parameters of the anisotropic slab which can be used to realize a given admittance dyadic. The relation is verified with results known for the perfect electromagnetic (PEMC) boundary and bi-axial material, considered as special cases of the theory