The PXE Specification (http://www.pix.net/software/pxeboot/archive/pxespec.pdf) specifies the following values for the client system architecture of the DHCP
0 = IA x86 PC(2)
1 = NEC/PC98(2)
2 = IA64 PC.(2)
3 = DEC Alpha (2)
4 = ArcX86 (2)
5 = Intel Lean Client (2)
While the list looks a bit dated, what do modern Architectures use for the Arch Field.
e.g. If I have a ARM Server or a RISC-V server and want to perform a PXE Boot what value should I provide for the ARCH Field?
From https://www.rfc-editor.org/rfc/rfc5970#section-6: "The following options have been assigned by the IANA from the option number space defined in Section 24 of the DHCPv6 RFC [RFC3315]."
So, I would suggest that https://www.iana.org/assignments/dhcpv6-parameters/dhcpv6-parameters.xhtml#processor-architecture is probably a good source.
Yes, this is DHCPv6, not DHCPv4, but no one appears to have cared enough to supersede rfc4578 with an updated version.
Having reached that conclusion, I peeked in UEFI 2.8. In Table 210. PXE Tag Definitions for EFI, the description for Client System Architecture is:
Type (2) Type is a two byte, network order, field that identifies the processor and programming environment of the client system. For the various architecture type encodings, see the table "Processor Architecture Types" at “Links to UEFI-Related Documents” (http://uefi.org/uefi) under the heading “Processor Architecture Types
That link in fact points to the IANA page again.