The ground is the one furthest from the 2wire plug. If the alt was built incorectly and the ground is not isolated it can be used in the alt1 position and move alt one to the alt 2 position.
Alt-1 is wired like most 12V systems, the ground lug goes to the ground post or frame ground on the front battery and pos goes to the pos post on the front battery.
Alt-2 is where the 24V system comes into play. It wires up like your back battery. Look how it is wired up. the ground on the back battery goes to the pos post of the front battery not to frame ground. The Alt-2 does the same, this is why it has to have an isolated ground "HAS TOO" no exceptions. If its ground is grounded to the frame, block, manifold, etc it can cause the batts to blow, you will ineffect short the front battery. Alt-2 pos lug goes to the pos post of the rear battery. The ground lug does go to the ground post of the back battery but remember that post goes to the pos post of the front battery. Thats why if its not isolated "Bad juju"