Thank you Hannibal for your answer!
On the other hand, what will happen while your army is fighting in a battle as a attacker and want send reinforcement in your stack for joining to your stack but not attacking the defender as a different stack ? ( I mean, join and attack in the next round)
For example, I was attacking with my stack and the enemy's allie came to help him as a attacker stack, so I needed more troops there. I wonder how can I send backup there without start attacking them as a separate stack??