Each domain which is registered and hosted has lots of DNS records - some of them are created automatically when you host a domain, others can be set up manually if needed. For example, the A record of a domain is its IP address i.e. this record reveals where the site is situated; the MX records reveal where the e-mails are handled, while through an SRV record you can use a domain for unconventional purposes, including a Voice-over-IP service. Being able to set up and modify certain records enables you to have different services with different companies if, for instance, your present host provides stable web hosting, but has difficulties with the email service, or you simply prefer to get a particular service from a specific company. Even though you should be able to set up and edit these records freely and without any restrictions as you own a given domain name, some companies charge extra to give you this option.