It isn’t. The difference is literally unchecking the “Enable resolution services” option in the Synology DNS UI. From Synology’s help topic on the setting:
You can enable resolution services to allow the DiskStation to resolve recursive queries. A recursive query occurs when the DiskStation is not authoritative for a requested domain. In this case, the DiskStation queries other domain name servers until the information is found, or until the query fails.
For example, if a client queries your DiskStation for the address of “synology.com,” but your DiskStation is not authoritative for this domain (i.e. you do not own synology.com), it will query other domain name servers or forward the request to specified forwarder servers, and then relay the result back to the sender of the query.