From the CST Help:
##Radar Cross Section (RCS)##
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:
###Total RCS:### TheThe total radar cross section is defined as the ratio of the scattered power to the intensity of the incident plane wave.
Total ACS:
###Total ACS:### TheThe 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)