This article focuses on best practices to configure Uplink Balancing and Multipath Rules for 2 or more WAN links.
Thanks to @BAlfson for great explanation about default multipath rule.
To distribute traffic evenly on 2 WAN links,
Target: WAN interface "WAN_200_225" is only used by 192.168.10.9 to access Internet, no other internal host/network can use it to access Internet.
Assume UTM has 3 WAN interfaces, traffic from an internal network needs to be loaded on 2 WAN interfaces only.
Sophos UTM: Uplink Balancing and Multipath rule, support.sophos.com/.../KB-000034635
Balancing by connection is a good way to enter into ReCaptcha hell and be forced to log in over and over. I use multipath for failover.