Commander
(Rule)Description: Detachments with the Commander special rule are the commanders of your Army.
When deploying a Formation that includes any models with this special rule during deployment, a player must assign all models with this special rule to a Detachment in that Formation, so long as the two of the same Detachment type. For the remainder of the battle, the model counts as part of the Detachment it is attached to an cannot leave it for any reason.
A Detachment can only ever have one model with the Commander special rule attached to it at any given time.
If, for whatever reason, a model with the Commander special rule cannot be assigned to friendly Detachment during deployment (e.g., due to there being no Detachment of the same type in the same Formation) then it acts as its own Detachment on the battlefield.