For example, when I have a parent task with 2 subtasks. Each subtask I estimate for 2 and 3 days. The parent task I add 5 days (this is the total est of 2 subtasks). Then, when I check the velocity, if Jira sum the estimation of both the parent task and subtasks to 10 days?
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No. Velocity does not look at the estimates on non-sprint items. Sub-tasks are a part of their parent story, not sprint items.
For Scrum (and hence velocity calculations), you do not put estimates on sub-tasks. Jira won't work if you do that.
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