The CO zone is an area around a CO unit in which a unit can receive the benefits of that CO. The radius of CO zones varies between COs, but the max size of a CO zone is 5. The radius of a CO zone increases one for every meter in the top right corner that is filled.
Any unit in a CO zone that is controlled by the side of the CO receives +10% offense and defense while they are in it. After that bonus, the effects that a CO gives certain types of units units in a CO zone is applied. Some COs only give the extra bonuses to certain units.
Damage dealt by a unit in the CO zone fills up the bar in the top right corner (with 1/12 of the bar being filled for every 5 damage in HP done). When it is filled halfway, the CO zone is expanded by 1 space. When it is filled completely, the CO zone is expanded by an additional one space (two spaces larger than the original in total, and a CO power becomes available to the player. The CO power may give additional bonuses to units (or, in Tabitha's case, harm enemy units), and the effects of the user's CO zone are applied to every unit controlled by the user.