Max Clearence is an upgrade available only to Allied Construction Yards and Command posts. Once researched, it opens the 3rd and final tier of the Allied Arsenal. However, a Defense Bureau is required to unlock some units. Note that the upgrade has only a local effect, meaning that if the Construction Yard has researched Max Clearence then it will only affect production buildings in its vincinity only. However, you can research the clearence via command posts themselves if you want to grant your production buildings with improved clearence at the cost of researching the same upgrade individually.