WHAT IS REFLASHING

you spoke of having While finding the maps in a raw hex file is possible if you have the right skill set, it's beyond the scope of this course and we will be assuming that you're working with a well developed and accurate definition file.

Where are these definition files available from please

Usually they are within your reflashing package, like HPTuners, ECUFlash, ECUTEK... That's why it's important to do some research before buying anything. Support is very important. I had a Subaru that I wanted to tune using HPTuners, and the definition file didn't exist for my ECU. I sent an email to their support department and after a couple days they sent me an updated software which included the definition file so I could read and modify the maps.