FreshTomato Wiki

Similarly to the Routing page the MultiWAN Routing allows you to affect traffic outbound in case of multiwan scenario. In this page you can define advanced parameter to make a connection “sticky” to a specific WAN. Directing traffic outbound will implicitly affect the return traffic as well; so a packet going to WAN0 is expected to return to the same interface only.

On: If enabled makes the rule active.
Protocol: All/TCP+UDP/TCP/UDP/ICMP - Here you can define the transport protocol that should be matched by the rule. Please not there are hundreds of layer 4 protocols this page and this setting in particular refers to the most common only.
Source Address: All/IP/MAC - Filters in the defined source of traffic. All stands for all LAN devices, IP allows you to define a specific IP or set of IPs in a netmask syntax (e.g. 192.168.1.0/24), where MAC binds the rule to a defined MAC address only.
Destination Address: All/IP/Domain - Similarly to the source IP the destination address makes the rule more restrictive based on/or destination address. This can be defined as ALL (as in Internet), IP or set of, defined in a netmask format or even a destination domain.
Selected WAN: WANx - Is the logical interface where the defined traffic should be sent to in outbound.
Description: Free text field to ease the management.

Please note the rule defined will take precedence over any WAN's weight related setting.