The ELIS account number is the number that you were assigned after registration on the USCIS Electronic Immigration System (ELIS). Note, however, that ELIS is the previous name of the online system provided by USCIS.
If you have registered online with USCIS before, you need to obtain your Online Account Number (previously, an ELIS account number), which you can view on your profile page after you sign into your account at USCIS. Your Alien-number (A-number) is different from the USCIS Online Account Number (ELIS number).