Commanders are special units that represent your hero's physical presence on the map. Mechanically, they function the same as an attaché, except that an army can host both an attaché and a commander at the same time.
Commanders & Army Units
When your commander is attached to an army unit, the army unit is considered its host. Like an attaché, the commander unit moves wherever the host moves.
A hosted commander can apply any skill related to the host's attribute when taking military action. (Story tags are also applicable.)
Capturing Commanders
Because commanders represent your hero, it's more difficult for a commander to be captured by the enemy.
If a commander is about to be captured, you may make an opposed Deft check (using your hero's abilities) vs. the enemy army to escape and move away 2 hexes. Add any skills or story tags that apply.
If you're captured, it's up to the GM to decide what happens next—for example, this may require the GM to employ a different game mode than the overworld to resolve.