--dhcp-pxe-vendor=<vendor>[,...]
According to UEFI and PXE specifications, DHCP packets between PXE clients and proxy PXE servers should have PXEClient in their vendor-class field. However, the firmware of computers from a few vendors is customized to carry a different identifier in that field. This option is used to consider such identifiers valid for identifying PXE clients. For instance

--dhcp-pxe-vendor=PXEClient,HW-Client

will enable dnsmasq to also provide proxy PXE service to those PXE clients with HW-Client in as their identifier.