Non authoritative servers do not contain copies of any domains. Instead they have a cache file that is constructed from all the DNS lookups it has performed in the past for which it has gotten an authoritative response. When a non-authoritative server queries an authoritative server and receives an authoritative answer, it passes that answer along to the querier as an authoritative answer. Thus, non-authoritative servers can answer authoritatively for a given resolution request. However, non-authoritative servers are not authoritative for any domain they do not contain specific zone files for. Most often, a non-authoritative server answers with a previous lookup from its lookup cache. Any answer retrieved from the cache of any server is deemed non-authoritative because it did not come from an authoritative server.
NON-AUTHORITATIVE DNS RESPONSES
DNS servers cache responses so that if another local user requests the same host or IP address from the local nameserver, the answer will already be in the local nameserver's DNS database. The server will not have to go outside the network to resolve that same host again.
See the section on authoritative responses for more information.