After WW2 at the Nuremburg trials for war crimes how was it decided who should stand trial.?

Apart from the obvious candidates like Goering,Himmler etc how was it determined whose conduct warranted a criminal trial.Hess for instance was in British custody after having flown to Scotland in 1942 in a probably misguided attempt to broker a peace deal yet stood trial in 1946 for war crimes.Did he indulge in activities which were serious enough to attract the attention of the Allies. Were there any rank and file German military personell indicted for war crimes.I am deeply puzzled as to why Rudolph Hess was incarcerated in Spandau prison until he died. If this question causes offence to any individual please accept my apologies.