The “For Tests” section of the TestRail for JIRA addon is to display TestRail results which have been linked through the defect field. With JIRA also integrated on the TestRail end, when submitting test results you’ll be able to use the Push link to quickly create new JIRA issues, the returning JIRA ID is auto-filled in TestRail linking the results to the new issue. You can also link to existing issues by simply filling in the defect field with a known JIRA ID.
The “For Cases” will also display test results for any TestRail cases that have the JIRA ID in the references field. Which method you use (or both) will depend on how you team prefer to operate.
I hope that helps clarify and let me know if you have further questions.