Each domain name which is registered and hosted has a lot of DNS records - some of them are set up automatically when you host a domain, others can be set up manually if required. As an example, the A record of a domain address is its IP address i.e. this record reveals where the website is located; the MX records reveal where the emails are stored, while using an SRV record you can use a domain for unconventional purposes, including a Voice-over-IP service. To be able to set up and modify certain records allows you to have different services with different companies if, for instance, your current host provides dependable web hosting, but has issues with the e-mail service, or you simply prefer to get a particular service from a specific company. Even though you should be able to set up and modify these records freely and without any limits as you own a given domain name, a lot of companies charge extra to give you this option.