A root cause records why an issue happened, chosen from a list the project maintains. It is what makes "how many of these were design errors?" a question you can answer at the end of a job.
This topic includes:
The default list
A new project starts with the causes most jobs need:
Design error · Design omission · Changed condition · Coordination issue · Damaged by others · Defective material · Installation error · Owner change · Weather · Other
Edit the list
Add your own from Issues → Settings. A new cause is appended to the end of the list rather than sorted among the defaults.
Set a cause to inactive to stop it being offered on new issues. Issues already recorded against it keep it, so past reporting is unaffected.
Tip: root cause is most useful filled in at closure, not at creation. When an issue is raised nobody knows yet why it happened; when it is closed, somebody does.