Hi again,

I see that principes and hastati can swap their positions, but why later legionaries can't? I'm not a Roman expert but I know that post Marian maniples were able to exchange their positions in combat in order to hold a permanent fresh troops line.

Thank you

Well this rule is to simulate the exchange of lines between ther hastay line and Principes line within the "Polybian Roman".
The army of this period always deployed on a tripla aces (triple line).

The reason of the adopting more lines worked with the enemies met by the Romans before Hannibal showed them that a longer front could be more effective. Of course a longer front makes more sense if you have more cavalry (so you can take more advantage by flank manouvres).

Scipio adopted a more linear deployment at Zama (where he also had more cavalry). From this moment the tripla aces was dead.
Yes also Caesar deployed his coortes with reserve, or on more lines according to situation, but it was a different thing respect the rigid deployment of the "Polybian" Roman army.

The problem is that wargamer benefit of the sense of then (is it the correct expression in English?). Actually the indeep deployment is not a great solution. In other words gamers (that can choose also to buy more cavarly) will always play like Scipio.
So the rule is to invite the gamer to adopt this historical formation getting at least an advantage from this.