Tatl-arc is the x86_64 overlay for ARCVM; see the tatl and project-termina overlays for details. This board is almost identical to tatl with the one difference that the guest kernel config points to x86_64_arcvm_defconfig rather than chromiumos-container-vm-x86_64_defconfig.
This board is only used to generate the guest kernel prebuilts, which are then manually checked in to the Android tree.