The center-system regime under which high-energy weapons work, exploited by Porthus to arm a behemoth in Earth orbit.
Enhancement zone guns and beams are highly regulated, high-energy weapons that would normally only function in the center system - so expensive, and so demanding of space, that even Syndicate security corvettes do not bother carrying them. That is precisely why nobody expects them above Earth.
Porthus ordered a full complement installed aboard the Homecoming Queen anyway, alongside enhancement zone suits, betting they would work where they should not. At the epilogue every one of those systems reads green across the board, making the behemoth very likely the strongest ship in orbit, and York confirms to Colonel Boomer that the enhancement zone suits appear online and working just like Porthus predicted, with the Cronus-class Power Armor online as well. The bet has turned a defenseless-looking transport into the spearhead of a war.
The system-wide restriction the resistance has quietly broken, arming itself where nobody thought it could.