One of the things that makes Simian Rising different from most games is the ability to control groups of systems with just one key, or to assign one system to multiple keys.
When the game was released, there was a way of accomplishing this, but I've since come to the conclusion that it was too obscure. It was too difficult to tell which systems were attached to which keys, which systems could be assigned to custom keys, and the drag and drop method in use was just too clunky.
Well, now those shortcomings have been addressed! I think this new method of assigning keys to systems is much better than the old one.
In build mode, if a system can be assigned custom hot keys, a new panel will show in the module detail window.
If you'd like your defensive field to activate with the "1" key and the "4" key, just check those boxes. If you'd like your offensive field to activate with the "2" and the "4" key, click that module icon, and check those boxes in the details window too.
Now you can turn your defensive field on and off with the "1" key, the offensive field on and off with the "2" key, and force both active with the "4" key. Much better than the old way, and it's easy to see how your ship is configured.