If a source route specifics that certain traffic should always use one WAN – what happens when that WAN interface is down?
If a source route specifics a certain subnet should always use a given WAN that traffic is forced out that WAN. For example, A source route rule can always route 192.168.10.0/24 traffic out the DMZ WAN interface. However if the DMZ WAN interface is determined to be down according to the WAN Failover app, then the source route rule is ignored and that traffic will go out the other WAN(s).