A story is usually a user generated issue that tells you in human terms what someone wants/needs. Sub-tasks happen when developers need to break up stories for various reasons, usually that they want to have several of them working on different parts of it, or there are distinct tasks within it (e.g.. dev, test, and sign-off)
Epics are groups of stories. A stories sub-tasks belong to it, and a story belongs to an Epic, so there's an automatic derived ownership their - a sub-task belongs to the Epic that its parent is in.
Backlog is not an issue type or relationship. It's a pile of stuff that needs doing. Mostly stories.
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