There are some units and structures in the game that trade mobility, their ability to fire, or other functionality for additional armor. Crab and Gauss are just two examples. I couldn't find a good reference that lists all of them in one place though. I was thinking of this kind of format:
Maximum defensive ability
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Unit: Felon
How to set up for max defense: Set firing mode to "hold fire."
Weaknesses/reasons to toggle fire mode: Depletes shield energy to fire.
Advantage of setting to "hold fire": Full power to shields.
Disadvantage of setting to "hold fire": No offensive damage.
Automatically enters a different mode on move command? No.