It's been a while since I had to to use the info but I seem to recall it's handy to know when you have a MEL on the extract or blower in the Avionics bay.
Open Loop means avionics bay ventilated from outside. Closed loop means means skin valves are closed and bay is cooled from air conditioned air on OVRD.
Something like that anyways. I'll come back to clarify if I find the details. I know I filed them somewhere because it was quite the confusing issue at the time.