As ECS was initially a script used to handle communications, an hot-key was included so several scripts can share the same keyboard shortcut to handle communications with different factions. But it can be used for other purposes.
Basically it allows you to call the script of your choice when the player targets an object filling the parameters you've used in the register script and press the ECS hotkey.
However if two scripts collides, only the first one registered will be called. If as an example, 2 scripts register the ECS key to all pirate ships, only one will be called. That's why you should always try to limit the ECS shortcut to objects with a given local variable. Or use the key for specific tasks (scan asteroids, target a particular combination of class / race).
Check the demo script to see an example.