This feature is only available for tanks that have been improved from within The Mechanic building.
After improving these tanks, you can activate or deactivate their flags from the deploy menu before attacking, where you can also select the tanks you want to attack with.
Note that you cannot select more than one flag in the same legion, and to choose a specific flag, the tanks belonging to that flag must participate in the attack; otherwise, you will not be able to select it. Additionally, each tank has its flag that cannot be changed.

Note: These flags may not appear even if you have selected them in some attacks, such as attacking a resource field, and they will not appear if you use an army skin.