as @RomanMD said the available option for traffic flows can be configured in Security & SD-WAN > (configure) SD-WAN & traffic shaping
Uplink selection > flow preference - L3/L4 filters
SD-WAN Policies > VPN traffic - for SD-WAN traffic, L3/L4 - L7 filters
Both configuration are a 'preference', if the specified uplink is down the MX will use the other uplink (if available)
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