If I have been asked for an additional document then do I just attach it or I need to communicate back as well (mail back?)
As per their website it says: https://immi.homeaffairs.gov.au/help-text/eplus/Pages/elp-h1470.aspx Confirming you have provided the requested information
If the Department has sent correspondence requesting information, you should attach it as soon as possible. When you have provided all the requested information, select the I confirm I have provided information as requested control. This lets processing staff know that the application is ready to assess. If you do not select this control, the application will be assessed on the information available after the time you were given to provide the additional information has expired.
However I do not see the “I confirm I have provided information as requested control” on my immiaccount.