Welcome @Neelsk74, to the community!
Your question refers to a board which, like the corresponding device, has unfortunately already been discontinued, so in principle no one at STMicroelectronics can help you any more.
However, if I remember correctly, VCC_BRG_SW was an auxiliary voltage that was more positive than V_INPUT, so that it could be used to control the gates of Q102 and Q103. At the top of the page with figure 8 from the UM2537, from which you apparently copied the schematic excerpt, you can find the two gate drivers that switch this small auxiliary voltage onto the gates via the pnp bipolar transistors Q100 and Q101.
Does this answer your question?
Regards
/Peter