Each domain which is registered and hosted has a lot of DNS records - some of them are created automatically when you host a domain, others can be set up manually if required. For example, the A record of a domain is its IP address i.e. this record reveals where the site is located; the MX records reveal where the emails are handled, while with an SRV record you can use a domain address for unconventional purposes, like a Voice-over-IP service. To be able to create and modify specific records permits you to have different services with different companies if, for instance, your present host provides stable web hosting, but has problems with the e-mail service, or you simply prefer to get a particular service from a specific company. Though you should be able to set up and modify these records freely and with no limitations as you own a given domain, many companies charge extra to give you this option.