It has been available since 25th Feb and is for applicants that used all five choices on their application but are holding no offers.
Extra gives those applicants a chance to maintain motivation by adding additional choices before clearing.
Over 7,000 applicants achieved a place through this last year.
You need to go to
There is a video you can watch to help you called 'How to use Extra.'
Any more advice, you should contact UCAS directly.