Essentially, this is a 240 volt manual transfer switch. Use for single purpose low amperage device where you need to alternate between 2 sources of line input. The diagram is a bit wacky in that is shows the L1 and L2 as line input where it should be load supplied to a device. A1 & A2 is one source of line input and B1 & B2 is another. An example would be if you wanted to power a single mini-split heat pump with grid power but have the ability to manually switch it to solar/battery input during a power failure. For the money this offers the ability to do something like that for a bargain price. You would need to provide an enclosure of some sort but even with that the cost is well below that of a prebuilt manual transfer switch.