lanmancz is partly right:
You need the little black placard for both manual and standby operation - not for auto.
Flying in manual is an absolute emergency, fallback mode. You want to somehow keep the aircraft pressurized - any movement of the thrust levers will change the influx of air, and you would have to change the position of the outflow valve - try it, it is fully modeled!
In standby you will set the CABIN ALT to departure elevation -200, then switch to FLT. During cruise you set the CAB ALT to the suggest value according to your cruising altitude. When descending, you set the CABIN ALT to landing elevation -200.
This pretty much mimics what the AUTO mode is doing, too.
Jan