Source MAC addresses: Select a MAC address list definition, describing from which MAC addresses the packets are originating. If selected, packets only match the rule if their source MAC address is listed in this definition. Note that you cannot use a MAC address list in combination with the source Any. MAC address list definitions are defined on the Definitions & Users > Network Definitions > MAC Address Definitions tab.
Source MAC address lists can be used in Firewall rules.
From the built-in help: