A shows the amount of fortification points and the required amount (trigger) for the system to be considered fortified. Anything above that is a waste time and efforts.
B shows the same for undermining. Again, anything above the trigger is a waste of time and efforts.
If both triggers are reached, the fortified and undermined state cancel each other out.
C shows the trigger on the diagram (I believe it's both for fortifying and undermining as those are shown in percents, so trigger would mean 100% for both).
D is a legend for the diagram, explaining how C is the trigger.
Guess, I'll have to add an interface part to my Popular Guide to Powerplay, huh?