For a given stationary max-stable random field $X(t),t\in Z^d$ the corresponding generalised Pickands constant coincides with the classical extremal index $θ$ which always exists. In this contribution we discuss necessary and sufficient conditions for $θ$ to be 0, positive or equal to 1 and also show that $θ$ is equal to the so-called block extremal index. Further, we consider some general functional indices of $X$ and prove that for a large class of functionals they coincide with $θ$. Our study of max-stable and stationary random fields is important since the formulas are valid with obvious modifications for the candidate extremal index of multivariate regularly varying random fields.