As I recall, they're numbered sequentially starting at midnight each day, and mistakes -such as track warrants begun but not issued, etc- are included in the sequence. This is why you can occasionally hear a DS start issuing track warrant number such and such, say scratch that, issue another right away, and the new number is a lot further down the sequence than the old one on account of many other items being issued by other DSs system wide between the first attempt and the second attempt. No doubt each RR has it's own numbering scheme, possibly with codes for subdivisions or regions. It has to do with the "C" of CAD (Computer Assisted Dispatching) recording everything all the DSs do. It doesn't matter how the numbering sequence is made just as long as duplicated numbers aren't issued on the same date.