Yes, this ID will be necessary to complete the CDX registration process. You may look up your company ID number at http://epa.gov/otaq/regs/fuels/rfs-list.xls (in Excel spreadsheet format).
Question and Answer was originally posted at: Questions and Answers on the Renewable Fuel Standard Program (PDF) (55 pp, 221 K, EPA420-F-07-041a, August 2007, About PDF)