CST MICROSTRIPES provides an efficient model for metal-backed absorbing panels. To model these panels in the normal way, we would have to divide the thickness into a very small spatial elements (cells). This is because the wavelength (and decay-length) in these materials is often small. Consequently the number of cells would be very large, requiring a lot of memory and a lot of CPU time. Furthermore, the small cells would force the simulation time-step to be small, requiring even more CPU time.
The absorbing panel model does not model the material in 3 dimensions, but replaces the panel by a frequency-dependent surface impedance. This surface impedance is modelled using a circuit which requires many times less memory and CPU time than the 3 dimensional model it replaces. Furthermore the circuit model has no physical size, and so does not affect the simulation time-step. This makes the CPU time several times less again.
The disadvantage of the absorbing panel model is that there is no communication through the body of the material between different points on the panel surface (no sideways propagation through the material). It's true that this will slightly change the angular dependence of the complex reflection coefficients. However, this change should be very small, provided only that the modulus of the wave-number |K| in the material is several times that in the air.
In Build, any body, whether a solid or an infinitely thin surface, may be designated as an absorbing panel. The body will then be treated as perfectly conducting metal coated with a thin layer of the specified thickness and electrical properties.