What is the difference to the scan, when selecting fat loss or muscle gain as the primary goal?

The specific goals are separated to fall in line with calorie requirements in each scenario.  For example, Muscle Gain requires you to be in a calorie surplus whereas Fat Loss requires you to be in a calorie deficit.  The focus requires adherence over a time period, either in a surplus or deficit to achieve the desired result.  Whilst it is possible to maintain or gain a small amount of muscle tissue whilst losing body fat, the scientific literature validates the different scenarios for both goals, which is why the goals are set the way they are.