From the CST Help:

> ##Radar Cross Section (RCS)##
> 
> The radar cross section (RCS) is a farfield parameter that determines the scattering properties of a specific radar target...
> 
> The RCS plot includes two **integrated quantities** which characterise the target:
> 
> ###Total RCS:###
> The total radar cross section is defined as the ratio of the scattered power to the intensity of the incident plane wave.
> 
> ###Total ACS:###
> The total absorption cross section is defined as the ratio of  the absorbed power to the intensity of the incident plane wave.

That said, **Total RCS** is defined as the integral of scattered power divided by the intensity of the plane wave, that is, by definition, the scattering cross-section:
$$\sigma_{\text{sca}} = \left(\frac{|E_0|^2}{2Z_0}\right)^{-1} P_{\text{sca}} = \left(\frac{|E_0|^2}{2Z_0}\right)^{-1} \frac{1}{2}\int\limits_{\text{around target}} \mathrm{Re}\left[\vec{E}_{\text{sca}} \times \vec{H}^{*}_{\text{sca}}\right]\cdot d\vec{s}$$
(here $E_0$ is the incident field, $Z_0 \simeq 377~\Omega$ is the free space impedance, $E_{\text{sca}}$ and $H_{\text{sca}}$ are the scattered fields).

Likewise, **Total ACS** is defined as the integral of *all energy flux* around the target, divided by the intensity:
$$\sigma_{\text{abs}} = \left(\frac{|E_0|^2}{2Z_0}\right)^{-1} P_{\text{abs}} = \left(\frac{|E_0|^2}{2Z_0}\right)^{-1} \frac{1}{2}\int\limits_{\text{around target}} \mathrm{Re}\left[(\vec{E}_{0} + \vec{E}_{\text{sca}}) \times (\vec{H}^{*}_{0} + \vec{H}^{*}_{\text{sca}})\right]\cdot d\vec{s}$$
(Absorbed power is the power which entered the target, but did not leave it. For a non-absorbing target ingoing flux should be equal to outgoing flux, so the intergal would be equal to zero)