Reference numbers are six-digit numbers that start with a number or A, B, or C. The numbers are recycled every calendar year, and the most recent year is the default. When the report loads, you can select that same reference number from previous years if it is not the year you are looking for.
Project numbers begin with "PR" and are followed by six numerals. These are small projects, usually created by service requests, and not capital projects.
Work Order numbers begin with "W" and are followed by six numerals. Work orders are usually created by service requests.