How I'm going to run it;
If parent unit is IN combat, same as detachment or NOT, and has more ranks = steadfast.
If parent unit is NOT in combat = not steadfast as there is no enemy to compare to.
That means, if my detachments are in combat with a different enemy, and it is in a combat with a horde 10 files deep while it is only two ranks deep, but the parent unit is steadfast at THE MOMENT OF COMBAT for the detachment = steadfast.
If at anypoint my parent unit is in combat and no longer has more ranks and files compared to its enemy, then the detachment will rely on its own ranks to determin steadfast.
Anyone else running it different?