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The Actual value of work completed is calculated by the sum of story points belonging to completed* issues (for team, role or individual) since the sprint launch.
The Expected value is based on the days passed since the sprint was launched. It sums the daily points defined for the participants based on the sprint-team definitions. The ticker shows where this value is expected to be.
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* By default the definition of issue completion is a Done status. Depending on your organization policy and Jira setup, the app supports other definitions including the use to Jira issue resolution. See app admin guide for more details.