Boarding a Unit
To board a unit the unit must spend a turn on its respective factory, airport, port or HQ. Once starting a turn on one of these buildings the unit can be selected, and a 'CO' option will appear. Boarding a unit does not cost a move for the unit and can move immediately after boarding completes.
Boarding costs half the unit's cost to complete, meaning its cost can vary from 750 War Funds to 14,000.
In levels without an airport, the HQ is the only way to board onto a seaplane.
Once boarded, the CO cannot leave the unit unless it is destroyed or deleted.
Benefits of Boarding
Boarding a unit immediately gives the unit max veterancy, increasing it to the unique rank. This rank alone gives the unit an instant +20% attack and defense boost.
Boarding a unit also importantly activates the CO Zone, increasing the attack and defense of units in the zone, depending on the commander. Destroying units in this zone also allows the unit to deploy its CO Power after moving once both bars are full, at the cost of the CO unit losing its attack opportunity.
In short, the minimum stat a CO Boarded unit will be 130/130. Any bonuses after that (terrain and/or CO Zone effects) will stack after.
Loss of a CO Unit
If a CO unit is lost the CO power gauge will reset back to empty, a turn will have to pass before the CO can board another unit.