There are 3 levels of hierarchy in Jira:
Level 1 - Epic
Level 2 - Task/Story/Bug
Level 3 - Sub task
Level 2 is a parent issue, Level 3 is a child issue.
To create a child issue you can use the ellipsis (...) in the top right and select 'create sub-task'. You can also use settings to convert a Level 2 issue into a sub-task (Level 3)
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