Forcefield Controls are found around or within almost every Alien Base and Cache. They act as locks and are each connected to a forcefield, preventing progression through the base unless the corresponding Alien Tablet is used. The majority of Forcefield Controls require a Purple Tablet to be deactivated, though there are three exceptions to this; the Control outside of the Lost River Laboratory, which requires the Orange Tablet, and the two Controls in the Primary Containment Facility, which require a Blue Tablet.
Data Bank Entry
This device matches no known technologies, and is likely alien in nature. Power is being routed via the terminal to the nearby forcefield. The technology is far beyond anything encountered before by the federation. Nonetheless there is some chance it functions like a regular lock, and only requires the correct kind of key.