DHCP - Clients via DHCP/DHCPv6 relay agent
XG Firewall already has the option to serve either as a DHCP server, or as a DHCP relay agent for another DHCP server. However, when using the XG Firewall as the DHCP server, there is no option to serve DHCP clients via a DHCP relay agent (i.e., when another device is serving as the relay agent). As a result, DHCP pools on the XG Firewall can only be configured using address ranges that are contained within the subnet range of the selected interface. This option is available when setting up address pools in UTM9.
William de Vos - IMPROVES B.V. commented
Please make it possible to configure both DHCP Server and DHCP Relay in the Firewall OS, perfect example is using DHCP Server for wireless/wired guests and DHCP Relay (internal domain DHCP server) for internal network.