Set notation for ACL matrix
The notation means:

For any $(s_i,o_j) \neq (s,o)$, $A'[s_i,o_j] = A[s_i,o_j]$.

The correct symbol seems to be $\neq$, &quot;not equal&quot;, rather than what you wrote.
The symbol $\forall$ means &quot;for all&quot;.
The notation $s_i$ just means &quot;element number $i$ in the set of rows&quot;, and $o_j$ is similarly &quot;element number $j$ in the set of columns&quot;. You could just as well have replaced $s_i,o_j$ with any two other symbols (other than $s,o$).