Combat Efficiency

Combat Efficiency is a percentage bonus applied to units in combat that increases those units' HP and Unit Attack by the same percentage.
It is different from HP bonus, Unit Attack bonus in the sense that combat efficiency percentages apply on top of regular bonuses. 
Example: If a unit has a base HP value of 1000, and two distinct HP bonuses of 10% and 5% respectively, the unit's final HP is calculated as 1000 x (1 + (10% + 5%)) = 1150.
By contrast, a Combat Efficiency bonus of 10% would apply on the value including bonuses, leading to a final HP value of 1150 x (1 + 10%) = 1265
It is unclear at this point how Combat Efficiency relates to HP Amplification and Unit Attack Amplification but official answers suggest they are different bonuses.