Units take time to hire. As such, when you wish to hire a Unit, or more than one, they are placed into Queues according to their class.
You have 5 Hiring Queues. These allow you to hire Light Infantry, Recon, Mechanized Infantry, Armor, and Combat Aviation Units.
Each Hiring Queue has a maximum length, which is defined by the Level of its Hiring Building (e.g. the Barracks). To increase the maximum Queue Size for a specific Unit class - i.e. the amount of Units that can be hired at once - you will need to upgrade the relevant Building.
You can hire multiple types of Units within the same Queue. For example, if your Maximum Queue Size for Mechanized Infantry Units is 100, you can select any combination of Mechanized Infantry Units that make up this number.
Units are separated in the Hiring Queue according to Unit type. Please note that if you try to hire a Unit of a type that is already in the Hiring Queue, the new Unit will be grouped with the first one rather than added to the end of the Queue.
Example: You begin hiring 5 Sekira Drones and then add 2 Wildcat APCs to the Hiring Queue. If you add more Sekira Drones to the Queue before all the original Sekira Drones are hired, they will be added to the initial group of Sekira Drones, rather than added to the Queue behind the Wildcat APCs.
After they are hired, Units must be assigned to your army. Units can be assigned as they are hired individually, or collectively when all Units of the same type in the Queue have been hired. You cannot begin hiring a different type of Unit in the same Queue before you have assigned all of the previous type of Unit to your Army.
Assigning Units is as simple as tapping the “Assign” button in the Hiring interface, or the checkmark on the bubble over the relevant Hiring Building.
Units that are hired instantly for Diamonds do not need to be assigned.