Is the Calibration Verification Number (CVN) unique across all cars?
The CVN is not unique across all cars, at least this is my understanding. As spelled out from this forum post:
CVN stands for "Calibration Verification Number", this is a fancy name for checksum. What the ECM (or TCM) does is add up all the numbers within each segment of the flash memory, the total of those numbers is what the CVN equals. This allows people with a TechII to quicky check the data in each segment is correct or unaltered (eg, the CVN matches a known program). If any data is altered within the segment then the checksum (CVN) will change or not match what it originally was.
This Wikipedia article states the CVN is set by the manufacturer so they can tell if something has been altered within the programming. It is the responsibility of the manufacturer to set the CVN values. Due to this, the broadest granularity of uniqueness would be at the manufacturer level, with a vehicle model (or platform - with options concerning the powertrain) being most likely.
Someone who tunes a vehicle can set these values to whatever they desire so they can easily tell what they've touched during the process.
The calibration verification number also known as the checksum is unique to the software version on the ECU not the model of car.
The software on the ECU has a checksum, sometimes there is one checksum where the whole program/flash code is added up or sometimes there is a checksum per sector of memory so there could be 12, 16 or more checksum values per file. The program side of the software then calculates the checksum its self and compares it to known values contained within the file.
Some engines and ECUs are used in multiple models and even manufactures. So if you have two vehicles with the same ECU with the same software version the checksum will be the same. But if there is only one difference in the software the checksum will be different.
So for example a BMW 118D and 318D which are exactly the same age would most probably have the same checksum. But if one has had a software update and one hasn't, the checksums will be different. Another good example would be; you could have a Vauxhall/Opel Vectra and a Alfa Romeo 152 with the 1.9TDCI engines, if produced at the same time they usually have the same checksum.
So to answer your question: No the CVN is not unique across all vehicles it is unique to the software version on the Engine ECU.
If you have intentionally modified the software on the ECU, you need to make sure the actual checksum values(s) equals the value expected by the software. This is done by changing the value the program side wants to see, so in simple terms there will be a part of the software which looks likes this:
'All data in the previous sector must equal 123xyz'
You would need to calculate the new checksum and replace '123xyz' with the new value.
The checksum value and verification is completely contained within the ECU so it doesn't matter if you swap two ECUs over from say 318 to a 118 (as long as you program the keys, immobiliser, VIN etc).
This is only a brief description of the checksum principle/process as I would need thousands of words to describe fully the relationship between the processor, flash, EEprom etc. But hopefully this answers your question.