BIND 9.9.8-S1 Release Notes (What's 'S' stands for?)

[stutiredboy]

BIND 9.9.8-S1 <https://kb.isc.org/article/AA-01307> was released, but
what's 'S' stands for?

I did not found any docs described this.

Can this version be used on production environment?

Thanks.
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