There’s very little information online about this unfortunately but this is what I found: If you’re starting a train that is electrically dead (cold start or whatever you want to call it), the very first thing you will do when entering the cab is to press the ‘Service Retention Cancel’ button which allows you to take control of cab. Pressing the ‘Service Retention’ button will turn rheostatic ventilation off and air compressors off. So it sounds like you press the ‘Service Retention’ button when you’re disposing a unit in a depot and you want all systems to be completely shutdown. That’s the only time I can think off when you would want to shut off all air compressors as your main reservoir will slowly drop to zero.