This trigger expression works as follows:
1. It can be triggered if the interface status is error.
2. `{$SDWAN.HEALTH.IF.CONTROL:"{#IFNAME}"}=1` - a user can redefine context macro to value - 0. That marks this interface as not important. No new trigger will be fired if this interface is down/error.
3. `{TEMPLATE_NAME:METRIC.diff()}=1` - the trigger fires only if the interface status was up to (1) sometime before.

WARNING: If closed manually, it will not fire again on the next poll because of .diff.