In the USA the manufacturer sets the model year. There is a cut off typically in August or September and cars built after that model year changeover get their title issued in the "new" model year. So you can have two cars on the sales lot both with Zero miles and identical to each other. However one was built in June 2020 and the other September 2020. The one in June will come with a title showing ownership in the car as a 2020 model while the one in September will show ownership title stating it is a 2021 model year.
What is the standard in Thailand. If a car is built in 2019 lets say June but by the time it is shipped to the dealer and eventually sold it is now February 2020 and the first time it is registered it is 2020. Is that car a 2019 or a 2020? Obviously the older the "model year" the less the car is worth even assuming all other features, and mileage of the cars are identical.