Mandelstam is an option for DiracTrace, OneLoop, OneLoopSum and TrickMandelstam. A typical setting is Mandelstam -> {s, t, u, m1^2+m2^2+m3^2+m4^2}, which implies s + t + u = m_1^2+m_2^2+m_3^2+m_4^2. If other than four-particle processes are calculated, the setting should be Mandelstam -> {}.
Overview, DiracTrace, OneLoop, OneLoopSum, TrickMandelstam, SetMandelstam.