RFC 4578                    DHCP PXE Options               November 2006

2.  Option Definitions
   There are three DHCP options [5] defined for use by PXE clients.

2.1.  Client System Architecture Type Option Definition
   The format of the option is:

                Code  Len  16-bit Type
               +----+-----+-----+-----+
               | 93 |  n  | n1  | n2  |
               +----+-----+-----+-----+

   Octet "n" gives the number of octets containing "architecture types"
   (not including the code and len fields).  It MUST be an even number
   greater than zero.  Clients that support more than one architecture
   type MAY include a list of these types in their initial DHCP and PXE
   boot server packets.  The list of supported architecture types MAY be
   reduced in any packet exchange between the client and server(s).
   Octets "n1" and "n2" encode a 16-bit architecture type identifier
   that describes the pre-boot runtime environment(s) of the client
   machine.

   As of the writing of this document, the following pre-boot
   architecture types have been requested.

            Type   Architecture Name
            ----   -----------------
              0    Intel x86PC
              1    NEC/PC98
              2    EFI Itanium
              3    DEC Alpha
              4    Arc x86
              5    Intel Lean Client
              6    EFI IA32
              7    EFI BC
              8    EFI Xscale
              9    EFI x86-64

   This option MUST be present in all DHCP and PXE packets sent by PXE-
   compliant clients and servers.

ipxe.org
DHCP option 93 	Client architecture
0 	Standard PC BIOS
6 	32-bit EFI
9 	64-bit EFI

You could use this to determine which version of iPXE to provide to a particular client.
For example, in /etc/dhcpd.conf:

  option client-arch code 93 = unsigned integer 16;
  if exists client-arch {
     if option client-arch = 0 {
       filename "bin/undionly.kpxe";
     } elsif option client-arch = 6 {
       filename "bin-i386-efi/ipxe.efi";
     } elsif option client-arch = 9 {
       filename "bin-x86_64-efi/ipxe.efi";
     }
  }

kubuntu.org
dhcp-boot=pxelinux.0
dhcp-match=set:efi-x86_64,option:client-arch,7
dhcp-boot=tag:efi-x86_64,bootx64.efi

Known client architectures
          Type                 Architecture Name                Reference
        0x00 0x00      x86 BIOS                           [RFC5970][RFC4578]
        0x00 0x01      NEC/PC98 (DEPRECATED)              [RFC5970][RFC4578]
        0x00 0x02      Itanium                            [RFC5970][RFC4578]
        0x00 0x03      DEC Alpha (DEPRECATED)             [RFC5970][RFC4578]
        0x00 0x04      Arc x86 (DEPRECATED)               [RFC5970][RFC4578]
        0x00 0x05      Intel Lean Client (DEPRECATED)     [RFC5970][RFC4578]
        0x00 0x06      x86 UEFI                           [RFC5970][RFC4578]
        0x00 0x07      x64 UEFI                           [RFC5970][RFC4578]
        0x00 0x08      EFI Xscale (DEPRECATED)            [RFC5970][RFC4578]
        0x00 0x09      EBC                                [RFC5970][RFC4578]
        0x00 0x0a      ARM 32-bit UEFI                    [RFC5970]
        0x00 0x0b      ARM 64-bit UEFI                    [RFC5970]
        0x00 0x0c      PowerPC Open Firmware              [Thomas_Huth]
        0x00 0x0d      PowerPC ePAPR                      [Thomas_Huth]
        0x00 0x0e      POWER OPAL v3                      [Jeremy_Kerr]
        0x00 0x0f      x86 uefi boot from http            [Samer_El-Haj-Mahmoud]
        0x00 0x10      x64 uefi boot from http            [Samer_El-Haj-Mahmoud]
        0x00 0x11      ebc boot from http                 [Samer_El-Haj-Mahmoud]
        0x00 0x12      arm uefi 32 boot from http         [Samer_El-Haj-Mahmoud]
        0x00 0x13      arm uefi 64 boot from http         [Samer_El-Haj-Mahmoud]
        0x00 0x14      pc/at bios boot from http          [Samer_El-Haj-Mahmoud]
        0x00 0x15      arm 32 uboot                       [Joseph_Shifflett]
        0x00 0x16      arm 64 uboot                       [Joseph_Shifflett]
        0x00 0x17      arm uboot 32 boot from http        [Joseph_Shifflett]
        0x00 0x18      arm uboot 64 boot from http        [Joseph_Shifflett]
        0x00 0x19      RISC-V 32-bit UEFI                 [Dong_Wei]
        0x00 0x1a      RISC-V 32-bit UEFI boot from http  [Dong_Wei]
        0x00 0x1b      RISC-V 64-bit UEFI                 [Dong_Wei]
        0x00 0x1c      RISC-V 64-bit UEFI boot from http  [Dong_Wei]
        0x00 0x1d      RISC-V 128-bit UEFI                [Dong_Wei]
        0x00 0x1e      RISC-V 128-bit UEFI boot from http [Dong_Wei]
   0x00 0x1f-0xff 0xff Unassigned