The project administrator  can choose which of the available issue types should be handled as defects, by dragging the available issue types to the corresponding target box. 

A defect represents an error, flaw, failure or fault in the SUT (System Under Test) that produces an incorrect or unexpected result. All the issue types mapped as Defect Issue Types are handled as defects.

Example of a possible setup

Don't forget to save the settings whenever adding a Issue Type to the "Defect Issue Types" box.