i dont know the exact terminology i just know how it works i deal with it day in day out.

a 24v system will have a + and - terminal joined between the batterys, a 12v will not.
a 12v will run side by side with no + to - joined between them.

i have cat d6 dozers that run 24v with the 3306 engine, i have a cat challenger that runs 3306 engine at 12v, they also used the 3306 engine in old phoeden lorries.