Before selecting an army you should refer to the force organisation chart for the scenario you are going to play. There is a different chart for Standard Missions, Battles, Raids and Breakthroughs.
The chart limits what forces are available to you. There are five categories of troop types in every army list. They are: HQ, Elites, Troops, Fast Attack and Heavy Support. One box on the chart allows you to make one choice from the equivalent section of your army list.
Dark boxes are required troop types. You must make a selection from the appropriate army list section for each dark box on the force organisation chart.