There are both sub accounts and virtual sub accounts. This is a minor distinction with major consequences.
What are sub accounts?
Sub accounts are a useful way of organizing risk under a master account. The master account can view and transfer funds in and out of sub accounts, but cannot make trades on their behalf. To make trades with a sub account you'll have to log in using that sub account's dedicated credentials, or connect it via API key to a 3rd party service. With Tuned, you might have the idea to dedicate one sub account to each of your live runs. That is fine, but only under the assumption that you have connected a regular sub account.
What are virtual sub accounts?
Virtual sub accounts do not have login credentials, and can only be controlled via API key. Should you run into problems, there will be no way to manually intervene. For this reason, we strongly advise against virtual sub accounts, and are in favour of regular sub accounts with dedicated login credentials.