a DOS Case ID if you attended an immigration interview at a U.S. consulate or embassy. So, if you are currently in the U.S. and have a pending I-485, then you would have an A-number but not a DOS case ID. So, do not worry about that unless you have an immigrant visa, and you are traveling to the United States as a lawful permanent resident, you will be required to pay an immigration fee before you can get your green card. You have to pay this immigration fee on USCIS online portal. But to do this, you need your Alien Registration Number (A-number) and your DOS Case ID.