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When a test case is assigned to you.
When your test case is modified.
When your test case receives a comment.
When your test case is moved.
When your test case is deleted.
When someone creates a new version of your test case.
When there are bulk operations of the any of the above (You will only receive a single email for bulk operations).
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If a bulk operation is performed on multiple test cases, the users will get an email with the IDs of the affected test cases:
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