APU ON 10th Stage OFF. After Takeoff (Pressurized meaning Packs ON): at 1500 ft or so you perform an "Overs" non verbal checklist which is APU START/STOP pressed (to Stop APU), L 10th Stage OFF, ISOL OFF, R 10th Stage OFF, LCV OFF then once your APU RPM drops below 50% you can then press PWR/FUEL. As Philip mentioned its also used in Cold/Moist Conditons where Bleed Air is a factor. Note: Its ALWAYS used during UnPressurized takeoffs (Packs Off) due to short field departures etc. I would suggest learning the APU and 10th Stage first so when the 14th is modelled its much easier to learn. Any more questions feel free to send me a PM. Tristan.