A standard query specifies a target domain name (QNAME), query type (QTYPE), and query class (QCLASS) and asks for RRs which match. This type of query makes up such a vast majority of DNS queries that we use the term "query" to mean standard query unless otherwise specified. The QTYPE and QCLASS fields are each 16 bits long, and are a superset of defined types and classes.
The QTYPE field may contain:
<any type> matches just that type. (e.g., A, PTR). AXFR special zone transfer QTYPE. MAILB matches all mail box related RRs (e.g. MB and MG). * matches all RR types.
The QCLASS field may contain:
<any class> matches just that class (e.g., IN, CH). * matches aLL RR classes.
Using the query domain name, QTYPE, and QCLASS, the name server looks for matching RRs. In addition to relevant records, the name server may return RRs that point toward a name server that has the desired information or RRs that are expected to be useful in interpreting the relevant RRs. For example, a name server that doesn't have the requested information may know a name server that does; a name server that returns a domain name in a relevant RR may also return the RR that binds that domain name to an address.
For example, a mailer tying to send mail to Mockapetris@ISI.EDU might ask the resolver for mail information about ISI.EDU, resulting in a query for QNAME=ISI.EDU, QTYPE=MX, QCLASS=IN. The response's answer section would be:
ISI.EDU. MX 10 VENERA.ISI.EDU. MX 10 VAXA.ISI.EDU.
while the additional section might be:
VAXA.ISI.EDU. A 10.2.0.27 A 18.104.22.168 VENERA.ISI.EDU. A 10.1.0.52 A 22.214.171.124
Because the server assumes that if the requester wants mail exchange information, it will probably want the addresses of the mail exchanges soon afterward.
Note that the QCLASS=* construct requires special interpretation regarding authority. Since a particular name server may not know all of the classes available in the domain system, it can never know if it is authoritative for all classes. Hence responses to QCLASS=* queries can never be authoritative.