It's essentially a checksum of the rom.
For any given calibration ID, theoretically there is an entry in their database that says an ecu rom with the calibration ID of, for example, A2WC522N (one of the 05 lgt roms), should return a specific checksum when polled.
When we go and tune the ecu for various things, whether it be fuel tables, MAF scaling or cel fixes for things like tgv deletes, we alter the output of the checksumming process.
If the cvn is actively being checked during the new smog process, things become a lot more involved for people with injectors, tgv deletes, and other mods that absolutely cannot run on a stock, unmodified rom.
But yes, essentially California vehicle nazis.