The Additional Basic Sum is seen as additional security during your stay in Germany which you might need to deposit into your blocked account. Usually, you do not need to deposit an Additional Basic Sum. In case you need to do so, your local authority which is handling your visa application will inform you about this. If no such request has been made, please do not enter an Additional Basic Sum while ordering the Blocked Account.
How can the Additional Basic Sum be released?
You have 2 options to get your Additional Basic Sum back.
1) With the written approval of the German authorities. If you choose this option please contact a German embassy or foreigners office and ask them to confirm in writing that this amount can be released. The document can say that you do not need an Additional Basic Sum blocked, or that the remaining amount on your blocked account can be released now.
2) With proof that you left Germany already. This can be your Abmeldung (address de-registration) form combined with a flight/train/bus ticket out of Germany.