• Stories, also called “user stories,” are short requirements or requests written from the perspective of an end user.
  stories are something the team can commit to finish within a one- or two-week sprint.
• Epics are large bodies of work that can be broken down into a number of smaller tasks (called stories).
  Teams often have two or three epics they work to complete each quarter.
• Initiatives are collections of epics that drive toward a common goal.

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Epics, Stories, Themes, and Initiatives | Atlassian