HRM newbie - trying to lose weight

I want to buy a HRM (I'm thinking of the polar ft40). I hear from one source that you need to work out in the fat burning zones, at a lower bpm, to burn fat... And others say that the higher the heart rate, the higher the calories burned, so you should work out near the top of your zone to burn the most calories.

Which is it? Why does a lower heart rate help you lose body fat? How about the range? Different sources give different bpm... And should I aim for bpm or percentage of max heart rate or percentage of heart rate reserve?

I'm so confused :sick: