Actually, There are three categories that you need to take care off before achieving goal of muscle gain or fat loss. The categories are:
i. Underweight or Low body Fat
ii. Overweight or High body Fat
iii. Underweight or High body Fat
These are the categories that we need to take care of. So let me explain point by point. Firstly we do have Underweight or Low body Fat. If your body is under first categories than you need to follow muscle building program. In this program sometime you  need to loose carbs but mean other time you have to mass up you body.
If your body falls under second category that is Over weight or high body fat than you need to follow fat loss program for almost 4 months and during this program, if you found you body to loose your muscle, over weight loss than you need to increase your carbs for few weeks and need to continue lean mode program.
If you are under third category, i.e, Underweight or high body fat than your body is in skinny fat mode. For this you need to follow three programs. At first you need to follow lean body program in this you will gain only muscle rather than fat. And secondly you need to follow muscular program. In muscular program you will loose your fat which are stored in your body. and finally you need to follow pure mass up program in which you will gain muscle to your body .