Horde Bonus is an advantage that some units of the Chinese army would earn when in a group of five or more, increasing the damage done by 25%. When Horde Bonus is active, there is a red star visible beneath the units.
It is unknown what this Bonus actually means. Possibly the troops gain confidence in battle, or simply the "strength in numbers" tactic.
Any combination of the above mentioned units would activate the Horde Bonus, even when garrisoned in buildings/units.