Xert records an estimate of the amount of fat and carbs you have used up in your activity. This information is available on the Activities Dashboard as well as on the Activity Details page for the activity. The software uses an algorithm based on your fitness signature, whereby the highest fat oxidation rate is achieved at your Lower Threshold Power and the oxidation declines as intensity increases. In addition, those with larger High Intensity Energy capacity are likely to use more carbs and fats at similar intensities to those with smaller capacity. Here is an example chart that shows what the calculation looks like:
Note that the amount of fat and carbs calculated does NOT include your basal metabolism. It is derived from the work you perform that gets recorded by the power meter.