Dali power injector circuit - an LM317(SOT223) + 5R1(2512) resistor, which will limit the current on the two DALI pins to about 240mA. This is a tried and tested circuit, but the LM317 will overheat if you try and deliver the full 240mA for too long, as would happen if you had a dead short on the bus, or a misbehaving device. Testing on a workbench, it dropped to about 130mA after a few seconds, but obviously that will vary based on the thermal environment.
The third part is an 0603 signal diode - there only to ensure that if this board isn’t powered, and power is supplied to the DALI bus from elsewhere, that the LM317 is protected from the reverse current. Most 0603 diodes are rated for 150mA, not 240mA, so it’s fair to say this board design does work, but hasn’t been stressed to the full 240mA output.
Note pin1 is labelled 12V but is really 12-20V, and pin2 is also labelled 12V but should be GND!
See DALI power board 3 for a revised version with a larger footprint LM317, for better thermals.