Mains (230v) is connected to "L" (live) and "N" (neutral) and the voltage regulated device is connected to "1" and "2".
Because it's not documented if "1" or "2" is the live wire, a test, with a mains tester screwdriver, on transformer-level 5, showed that "1" is the live wire.
Out of curiosity I took a look inside the transformer. Here I couldn't figure out how the white wire ("1") is the live wire and the grey is the neutral.
How does electricity flow in this transformer?
Documentation from the manufacturer:
