The BIND9-S edition is distributed to ISC Support Subscribers at the Silver level and above.
A recent version of the Administrative Reference Manual is shared here, to facilitate understanding of what the -S Edition offers. If you are using the BIND9-S Edition, please instead consult the relevant version of the ARM, included in the distribution.
The file below is an epub format archive of the ARM for BIND version 9.20.25-S1.