Can someone explain me this part of the schematic?
The purpose is to have a correct regulation at near zero volt output. But in my opinion the circuit is wrong, as it should pull the current below zero voltage and the power BJT transistor should just compensate that leak.
EDIT:
Perhaps you will find some information in this answer, how this simple/non-precise current sink works. But your circuit can't do the function it was planned for. Without a current sink, you won't be able to control low voltages. If introduced,  it has to be supplied also with negative voltage, so that  it keeps the main BJT always conducting a &quot;little bit&quot;.

EDIT 2:
After user Bruce Abbott comment, I see that original schematics is OK, I didn't see the detail. So forget about my &quot;correction&quot;