Every single domain name which is registered and hosted has lots of DNS records - some of them are set up automatically when you host a domain, others can be created by hand if needed. For example, the A record of a domain is its IP address i.e. this record shows where the web site is situated; the MX records reveal where the emails are stored, while with an SRV record you can use a domain name for unconventional purposes, for instance a Voice-over-IP service. To be able to set up and modify particular records permits you to have different services with different companies if, for instance, your current host provides good web hosting, but has troubles with the e-mail service, or you simply prefer to get a particular service from a certain company. While you must be able to create and edit these records freely and without any limitations as you own a given domain, many companies charge extra to give you this option.