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When NUnit executes, the output will be stored in this file. Even when specifying multiple files to test, all of the output it put into a single file. The output file specified is relative to the agent's workspace so to reference the output directory of the workspace, you only need Output\ (not $Workspace$\Output). However you can still specify an absolute files file location such as C:\myTests myOutputFile.xml and the NUnit action will generate the output file in the correct location.
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