The interface blocks
The interface block encompasses every subsystem that provides the user with feedback, being visual (screens, status LEDs, and so on) or otherwise haptic feedback. Just like the actuator block, these can be leveraged as (usually high-speed) exfiltration interfaces. Depending on how you look at it, you can classify the interfaces here that are targeted at pure point-to-point physical/bus communication (serial, USB, and so on).