Traffic will be routed through the healthy WAN connection that has the highest priority and is not in full load of downlink bandwidth. When this connection gets saturated (95% of defined download bandwidth), new sessions will be routed to the next healthy WAN connection that is not in full load.
The overflow outbound policy is not available to cellular WAN connections. Because available speeds can fluctuate extensively it is not possible to calculate when a cellular WAN is nearing full capacity.
Common use case
Prevent traffic flow from slowing down when the connection runs out of available bandwidth.