Each domain which is registered and hosted has a great deal of DNS records - some of them are created automatically when you host a domain, others can be created manually if required. As an example, the A record of a domain address is its IP address i.e. this record shows where the web site is situated; the MX records reveal where the e-mails are managed, while through an SRV record you can use a domain address for unconventional purposes, like a Voice-over-IP service. Being able to set up and modify specific records allows you to have different services with different companies if, for illustration, your present host provides stable web hosting, but has issues with the e-mail service, or you simply prefer to get a specific service from a certain company. While you should be able to create 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.