Just a shot in the dark, others more knowledgeable will know. I offer it from the depths of my memory.
On the B747 Classic (just a Boeing) if you forced the APU electrics on in flight by closing the APU electrical field switches and selecting the APU electrics ON you would place the airplane in GROUND MODE (in flight). i.e pressurisation system would drive outflow valves open, take off configuration would sound with thrust levers forward etc. Therefore there was a procedural limitation to forbid this.
For info this characteristic was provided for engineering use on the ground during checks with the airplane on jacks.