  A surface-link is a closed surface embedded in the 4-space, possibly
disconnected or non-orientable. Every surface-link can be presented by the plat
closure of a braided surface, which we call a plat form presentation. The knot
symmetric quandle of a surface-link $F$ is a pair of a quandle and a good
involution determined from $F$. In this paper, we compute the knot symmetric
quandle for surface-links using a plat form presentation. As an application, we
show that for any integers $g \geq 0$ and $m \geq 2$, there exists infinitely
many distinct surface-knots of genus $g$ whose plat indices are $m$.
