RINs are not generated at the beginning of a month. Rather, gallon-RINs must have been generated by the time a volume of renewable fuel is transferred from the producer or importer to another party (at which point the RINs are assigned to that volume). See regulation Section 80.1126(e)(2). A producer or importer of renewable fuel can decide on its preferred approach of designating batch numbers (the BBBBB code in the RIN), and then put this batch number into all gallon-RINs generated up to a calendar month's production or 100 million gallon-RINs, whichever comes first.
If a producer/importer had predefined a specific volume limit (such as 8 million gallons) for each batch, and this limit was exceeded in the middle of filling a rail car, the producer/importer would have two options. Since the 8 million gallon limit is self-imposed, he could simply exceed this limit for this particular batch. Alternatively, he could generate a new set of gallon-RINs having a new batch number for the volume that exceeded 8 million gallons. In this latter case, the renewable fuel in the rail car would be transferred along with two separate batch-RINs, which differ only in the batch number (i.e. the BBBBB code) and the number of gallon-RINs they represent.
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)