What is the difference between is fail of and is failed by
I think it is a custom 'issue link' set up by your jira admin.
I suppose one is an inward link and the other is an outward link. Refer to https://confluence.atlassian.com/adminjiraserver073/configuring-issue-linking-861253998.html for more about issue links.
How you use it in terms of interpretation is related to your way of working..
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