An SRV (service) record is responsible for informing users of special services running under your domain.
For example, massivedns.com could have FTP access. So, instead of users manually entering the FTP details into their FTP client, the details can be stored directly in the domain name.
SRV records are accepted in the following format:
Format: _servicename._tcp.domainname.com (e.g. _ftp._tcp.massivedns.com)
Priority: 10 (if multiple SRV records are created, specify the priority)
Port: 21 (e.g. the FTP port)
Destination: domainname.com (e.g. massivedns.com)