Outbound NAT is legacy, its rules should be migrated to SNAT and it will eventually go away completely (use the search):
https://forum.opnsense.org/index.php?topic=52261.0
The Outbound NAT page is essentially only there because the setting for Automatic, Hybrid, Manual and Disable NAT was still there, which is admittedly (and obviously) confusing. The last changes duplicated the mode setting to the SNAT page as well, so you can ignore Outbound NAT and have no rules left there (i.e.: migrate them to SNAT).
https://forum.opnsense.org/index.php?topic=52261.0
The Outbound NAT page is essentially only there because the setting for Automatic, Hybrid, Manual and Disable NAT was still there, which is admittedly (and obviously) confusing. The last changes duplicated the mode setting to the SNAT page as well, so you can ignore Outbound NAT and have no rules left there (i.e.: migrate them to SNAT).
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