a record of what fuel you have used and where it came from and when.
i created a form for ourselves, so when we get some wood we fill a form in, pop it in the folder (supposed to be kept near the boiler)
Good luck! Ours was very simple, he looked at meter readings fuel receipts and the building where the heat was been used. His only suggestion was that we should record the heat meter reading monthly instead of just when we submitted the readings.