[hazeii]

That's pretty much it; although at higher intensities the calories per unit time is higher, the higher the intensity the more comes from short-term stores (glycogen) than from long-term (fat).

The numbers suggest 60 to 70 per cent of max heartrate (which is pretty easy) is the sweet spot for general fitness and fat burning (i.e. weight loss). For improved endurance, spend a lot of time in that zone; for improved performance spend shorter periods (well) above it.