SRV record
Service location: this protocol on this domain lives at that host and port.
SRV maps a service name (for example _sip._tcp.example.com) to a target hostname, port, priority, and weight. SIP, XMPP, some directory lookups, and a few modern protocols use it so clients do not hard-code ports.
The target must have A or AAAA. People publish SRV and forget the address records, or they point at a CNAME. Clients that do not speak SRV will never see it.
See also: A record, CNAME, DNS.
Fact source: IETF RFC 2782, SRV.