Page "Preboot Execution Environment" Paragraph 24

After receiving the requested extended DHCPACK, the Network Bootstrap Program is uploaded into RAM and after it is verified or if verification is not required, the NBP will be executed.

It has access to the APIs of the PXE firmware extension ( Pre-boot, UDP, TFTP, Universal Network Device Interface ( UNDI )).

Its functions or tasks are not described in the PXE specification.