On the 70 D this lamp was on whenever the cranking ignition was switched on.
I believe the later 720's & 730's the lamp was on only when the cranking engine ignition switch is on but the cranking engine is not running! Once the cranking engine has oil pressure a pressure switch turns the lamp off. After the diesel is running and you turn off the fuel to the cranking engine letting it run out of gas & stop, the lamp comes back on to remind you to turn the ignition off. It's so easy to ignore and forget that lamp on the 70 that I've made it a habit to just NOT drive it until the cranking engine has stopped.