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            <para>
              Specifies a list of addresses which require responses
              to use case-insensitive compression.  This ACL can be
              used when <command>named</command> needs to work with
              clients that do not comply with the requirement in RFC
              1034 to use case-insensitive name comparisons when
              checking for matching domain names.
            </para>
            <para>
              If left undefined, the ACL defaults to
              <command>none</command>: case-insensitive compression
              will be used for all clients.  If the ACL is defined and
              matches a client, then case will be ignored when
              compressing domain names in DNS responses sent to that
              client.
            </para>

The first paragraph says that it "Specifies a list of addresses which require responses to use case-insensitive compression." while the second paragraph says the opposite, that when the ACL has a value of "none" (meaning that no addresses match) that "case-insensitive compression will be used for all clients."

Based on a code analysis by @pspacek, I believe that the extent of the inconsistency may be the phrase "If left undefined, the ACL defaults to none: case-insensitive compression will be used for all clients.", which seems like it should read instead "If left undefined, the ACL defaults to none: case-sensitive compression will be used for all clients."

Testing is recommended to validate behavior before making changes.