Links configuration defines where to store links between Gantt chart tasks. Select None if you don’t plan to use a particular kind of link. The minimum setup usually requires at least End to start link.
Hierarchy link is need for child / parent relationships between arbitrary tasks. Multiple levels of hierarchy are supported. Jira sub-tasks are always shown as children of their parent issue, and don’t need Hierarchy link setup for this.
Starting app version 2.1.20, a direction can be defined for configured links, outward or inward. For instance, "Duplicate" link has inward direction 'is duplicated' and outward direction 'duplicates'.