Each domain that 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 by hand if necessary. For instance, the A record of a domain address is its IP address i.e. this record shows where the site is situated; the MX records show where the e-mails are handled, while with an SRV record you can use a domain address for unconventional purposes, such as a Voice-over-IP service. To be able to set up and modify particular records enables you to have different services with different providers if, for instance, your current host provides good web hosting, but has problems with the email service, or you simply prefer to get a certain service from a specific company. Even though you should be able to set up and modify these records freely and with no limitations as you own a given domain address, a lot of companies charge extra to give you this option.