If you require any further help, please feel free to message me. I am a qualified NVQ Training Assessor and have been for the past 5 years.
The answers you have recieved are correct, an Internal Customer is anyone of your colleagues, be it in your department or another, who you need to provide a service or product for, for example giving them information to enable them to do their job, you are providing them with a service.
Just keep your answers to the point and simple. Which level are you currently doing?