The term “Strategic Unit” refers to any Unit that receives a statistical boost when deployed with supporting Units. The type of supporting Units needed to achieve this Bonus will depend on the Strategic Unit in question.
To reach its Maximum Bonus, a Strategic Unit must be supported by the full number of supporting Units. This number differs for each Unit, but is stated in the Unit description.
Strategic Units are among the most powerful in the game. They cannot be hired at your Base, but can be acquired for Diamonds at the Store.