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The project administrator can choose which of the available issue types should be handled as testable/coverable entities , by dragging the available issue types them to the corresponding target box. 

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Note that you can also cover your defects (e.g., Bug) with tests, as long as you configure them as being "Coverable Issue Types".

Example of a possible setup

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When this option is enabled, Sub-Requirements are associated with Requirements by Sub-Task Issues. When this option is disabled, Sub-Requirements are associated with Requirements with issue links configured link configurations below.

Issue Links relations

A specific Issue link type for the Requirement -> Sub-Requirement relationship.

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Select the Link Direction for the parent -> child relationship of the Requirement -> Sub-Requirement.