This is a transformer for step-up, step-down and split phase output balancing purposes. The step up and step down part is easy: Input can be 120 Volts and output 240 Volts or input can be 240 Volts and output 120 Volts. Regardless of the input voltage the input current is limited by the built in breaker to 100 Amps. The split phase output balancing is a bit more complicated to understand: When the input is 240 volts you can have two 120 volt load groups with a total output of the full 100 Amp rating regardless of whether the loads are balanced or not. The transformer automatically takes care of the load balancing. See the links below for more details:
The Autotransformer is pretty complicated. I tried to explain a bit more about it in my Blog Post on the subject.