Why there is an Arrow mark on defense vehicle number plates ?

Defense vehicles have a unique numbering system and they are registered by the ministry of defense, Government of India. The upward arrow is always either first or the third digit. The two other digits of the first three digits signify the year in defense department procured the vehicle. The next is the base code, followed by the serial number. The letter ending after the serial number indicates the class of the vehicle. Why arrow? To prevent the number from being read wrongly in case the number plate or vehicle turns upside down.