This breach type triggers when the issue is X hours before its Due date.
Note: this will not work if the Due date field is not used on the issue. Make sure that the issue field configuration in the relevant project(s) use Due date.
"Due X hours from now"
When choosing this breach/trigger type for a SLA Target, you select an amount of hours (the X above) before the due date, that the SLA Target should breach. Note that the hours are in decimal, so if you enter "0.5", this means 30 minutes.
The Next breach field and the SLA tab on the issue will calculate the estimated time of breach based on this, and the time in the Due date field on the issue.
(for example, if X is 0.5, and the issue has a due date of 2020-09-30 at 11:00, then the SLA Target will breach on 2020-09-30 at 10:30, assuming the issue has not been completed before that (or the SLA Target has been cancelled by another reason such as switching the issue to another SLA).
Important: This type of SLA Targets can only trigger when the estimated breach time is in the future. You cannot set this SLA Target for an issue with a due date in the past, it will not trigger a breach.
Also, be careful when setting due dates in the near future. For example, if you set the SLA Target to trigger 10 hours before due date, and then create a new issue with a due date just 2 hours from now, it will not work. The trigger date would be in the past (it would be at -8 hours), so this SLA Target would not trigger/breach.