What is a document number on your license

This tends to come up with New York driver's licenses, so I am going answer assuming you are in New York.  The document number is an 8 or 10 digit alphanumeric number found either on the back of the license or lower right hand corner (depending on the year of your license).   It is not the same as the drivers license number.  I have included a link to the NY authority that displays images below.

https://dmv.ny.gov/id-card/sample-photo-documents