Documentation for JIRA 5.1.x. Documentation for other versions of JIRA is available too. 
In the default JIRA workflow, issues have their resolutions cleared upon re-opening an issue. This is important because many reports/filters could be inspecting for the presence of a Resolution to be considered resolved.
The Resolution field is typically cleared by setting a post-function in the workflow transition you'd like to have this occur. The function to use is Update Issue. The field to use is Resolution. The value to choose is None.
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