The difference between 15V and 15V transformer is mainly used at voltage. Hence, with a 15V voltage regulator, it is important to note that there are two main varieties of the two.
Both 12V transformer are used to operate differently. On top of that, the 15V transformer is also different as it is powered with a voltage input current (15V), 15V transformer is differently shaped and can be used with a power supply from 5V to 12V. When it comes to a car, 15V transformer is differently shaped and can be used with any power supply.