The SSCC18 really has 20 digits. The first two digits in this case are the application identifier. It tells the application that the 18 digits following the application identifier have this structure (if one is specified) and this meaning. Generally speaking you do not move the application identifier to your database, just the 18 digits. You can read all about this (both UCC/EAN 128 and the SSCC18) if you visit their web site.