On the documentation, I went looking and I do not think it exists.
The modes get named but never explained. The closest thing is the backup modifier tags section, which tells you how to override the mode for one VM with a tag and assumes you already know what the modes are: https://docs.xen-orchestra.com/xo5/backups#backup-modifier-tags. That is a gap on our side rather than something you missed, and it is written down now.
One thing before you make those changes. Switching to normal snapshots and powering off the replicas will probably make this morning's failure impossible to reproduce, and nobody has looked at it yet. If you can spare one more run in the current configuration, the VM_BAD_POWER_STATE on a delta job that worked for months is the interesting part.
If you would rather just get your backups working, do that instead. You have already spent enough of your week on this.
Also, correcting myself again: I tagged Team-XO-Backend earlier and that was the wrong team. A XAPI error should go to the storage side, because XO only calls into XAPI rather than implementing it. @Team-Storage, if anyone has a moment for the error in the screenshot at post #3.