The two terminals are connected to the socket of 220-volt power supply. The three lines are the output of the buck. The middle line and the two sides of the line are the same voltage. If the output of your transformer is double 12 volts, then the three middle lines and the next two will be double 12 volts.
There are many kinds of controlling transformers. There are usually BK-50~300 type low voltage power supply for machine tool control and lighting circuit, and larger power (BK-400~1000) for power supply of safety lighting.
The primary (high voltage side) of the transformer usually has two kinds of power supply voltage input for `~220 V and ~380 V. The voltage of the output end of the secondary (low voltage side) has a lot, which are 6.3 V, 12 V, 15 V, 24 V, 36 V and so on. 6.3 V is commonly used in indicator light power supply, 12 V, 24 V are used for intermediate relay power supply and controlling circuit power supply. 36 V is often used for lighting power supply.
Wiring: the zero terminal of the high voltage side is the public end, and when the power supply is 220 V, it is connected to the two terminals of 0 and ~ 220 V. When ~380 V is supplied, two terminals are connected to 0 and ~380 V. The machine tools use 380 V, with the same power, the power supply voltage is high, the primary current is smaller, so that the choice of the wire is smaller, according to this formula I = P/U. The end of each end shall not be faulted according to the marked voltage level. After the fault, the output voltage will not meet the requirement. If the low voltage is connected with high voltage, the transformer will be burned.