For example for a typical website:
If they all just need to point to one address then 'web forwarding' needs to be used, eg. when you type in mycompany.co.uk it auotmatically takes you to mycompany.com rather than displaying the same website on two different addresses which is confusing to Search Engines.
However sometimes you might want to have say firstname.lastname@example.org and email@example.com because they'll be reaching different people. If this is the case then please just raise a support ticket and we will 'map' the domain names you need onto the hosting account.
Just to be clear though for Shared Hosting this can only be used for one website, not to put multiple websites onto the same account as this causes imbalances in the hosting. If you need multiple websites on one hosting account then please consider Virtual Private Hosting or a Dedicated Server.