I was recently required to submit information to the TSA for a Threat Assessment / Background Investigation. In the Personal Information Section, the online application asks "Do you posess a current DHS/TSA Credential?" If you have a TWIC Card, it asks for the "Credential Reference Number". I looked all over the internet to find which number on the card they are looking for, finding nothing...so I called the Helpline and they told me what number on the card they were looking for. On the back of the card, under the bar code, on the left hand side is an 8 digit number that starts with a zero. Ignore all the other numbers. Hope this helps. Take a look around this message board, you might like it here.