Some plugins copy source code files to Jenkins' build folder so that these files can be rendered in the user interface together with build results (coverage, warnings, etc.). If these files are not part of the workspace of a build then Jenkins will not show them by default: otherwise sensitive files could be shown by accident. You can provide a list of additional source code directories that are allowed to be shown in Jenkins user interface here. Note, that such a directory must be an absolute path on the agent that executes the build.