You may have one Aid Center and a maximum of 5 Aid Stations at your Base. The Aid Center and each Aid Station are split into two parts: Emergency Revive and Diamond Revive. These function as follows:
In the Emergency Revive tab, you can use Resources to revive Units lost while defending your Base over the last 7 days. The amount of Units you can revive this way depends on your Aid Stations’ Power Limit. As with the Bunker, the more you upgrade these Buildings, the higher the Power Limit, and the more Units can be revived using Resources.
In the Diamond Revive tab you can use Diamonds to revive some of the Units you've lost during recent Battles. Units will disappear from the Revive tab after 7 days.
The amount of Units you can revive is limited by your Aid Stations' Queue Size and Power Limit. Research and upgrade the "Aid Station Queue" Tech to revive even more Units.