Depending on your contract, you have a certain number of free included domains available. You can view these online at any time in 1&1 IONOS: Choose a contract and select Manage Contract. In the Domains section of the Included Features page, you can see how many included domains are available and how many you are currently using.
If you delete an included domain or transfer it away from the contract, a new included domain is not made available automatically. However, you can contact 1&1 IONOS Customer Support to have a previously used included domain replaced by a new domain once per year.