The Jira users intuitively expect a hierarchical visualization of other entities like Sub-tasks, Epics and Porfolio hierarchies. Strictly, they are not links. However, Links hierarchy converts them into "real" links as expected by the users.
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Issue (Epic Link: <Name>) → Epic
Portfolio
Links Hierarchy supports Portfolio Hierarchy:
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As opposite to Jira Software, Portfolio does not support any customization on its GUI to integrate third-party content. Therefore you cannot view issue hierarchies from any portfolio page. However, Links Hierarchy is a fantastic way to spread Portfolio hierarchies everywhere. Everybody can see them integrated with issue hierarchies wherever Liks HIerarchy is displayed: issue view and board detail view. So the users can see their issues within the Portfolio context and managers can see unexpected impacts (linked issues, etc.) on the Portfolio hierarchy progress.