Instructions for configuring Vendor-Specific PXE Forced Mode can be found on HOWTO7071. With the introduction of UEFI replacing the BIOS in computers, different boot files are required to boot UEFI. To boot a BIOS system, a .0 file is required (bstrap.0), to boot a UEFI system, a .efi file is required (bstrap.efi); BIOS cannot boot .efi, and UEFI cannot boot .0.
In the Vendor-Specific PXE Forced Mode configuration, DHCP option 67 is used to inform the PXE Client what boot file to download and boot off of. Because Microsoft DHCP and most DHCP appliances (at the time of writing) don't support scripted DHCP options, this DHCP Option 67 needs to be statically set to either BStrap\X86pc\bstrap.0 or BStrap\X86pc\bstrap.efi, it cannot determine whether the client is booting UEFI or BIOS and provide a different boot file for each system.
The Symantec (Altiris) PXE Server service is unable to dynamically provide the boot file, and thus is unable to manage a mixed BIOS/UEFI network. Therefore, in order to support mixed BIOS/UEFI PXE clients in a network, either IP Helpers or a DHCP server with scripting capabilities is required. One example of a DHCP server with scripting capabilities is isc-dhcp-server (a common DHCP server on Linux).