Treat WAN priority as big clunky levers that open or shut a WAN link for use by everything else. It is responsible for WAN availability.
With no other configuration but this on a Peplink, user traffic will be load balanced all healthy WANs available in the highest priority. All other links will be unavilable.
If all WANs in P1 are unhealthy the WANs in P2 will be used. etc etc.
Dashboard WAN prioirty is usually used for least cost routing, the availability for each WAN in each priority is based on WAN health status (using whatever healthcheck is set on each one).
WAN priority in SpeedFusion is for SF traffic only. You can have a different set of priorities in the SpeedFusino config, but SF can only use the WAN links that are available to be used.
Outbound policy priority can be used to direct traffic over WANs or SFC tunnels in an order of preference. These are rules for application / user traffic flow more than about controlling the use of WAN links.
If you want to use all the WANs in a SpeedFusion tunnel they will need to be healthy and ‘available’ - so all in the currently live priority (normally P1).
Then you can decide to prefer one WAN over another using SpeedFusion priority. This then provides hot failover between those WANs.
As above. Dashboard for availability, Speedfusion for Speedfusion flow control, Outbound policy for application level control.