On the support $t$-designs of extremal Type III and IV codes

Let $C$ be an extremal Type III or IV code and $D_w$ be the support design of $C$ for a weight $w$. We introduce the two numbers $\delta (C)$ and $s(C)$: $\delta (C)$ is the largest integer $t$ such that, for all weights, $D_w$ is a $t$-design; $s(C)$ denotes the largest integer $t$ such that there exists a $w$ such that $D_w$ is a $t$-design. In the present paper, we consider the possible values of $\delta (C)$ and $s(C)$.