An additional domain refers to an additional website that can be hosted on the same web hosting account as the main website. This allows the user to manage and maintain multiple websites under the same account.
An additional container refers to an additional virtual environment isolated from the main server in which an application can be launched and run. In the context of containerization (e.g. Docker containers), this can be a new instance of an application that runs in a separate container.
Therefore, the difference between an additional domain and an additional container is that an additional domain is associated with the web hosting account and represents an additional website, while an additional container usually refers to an isolated virtual environment with its own resources where an additional application can be launched and run.