I managed to find the documentation for it as well in the Snippet generator:
If the total number of issues is greater or equal than one of the specified thresholds then a build is considered as unstable or failed, respectively.
the old plugin:
If the number of total warnings is greater than one of these thresholds then a build is considered as unstable or failed, respectively. I.e., a value of 0 means that the build status is changed if there is at least one warning found. Leave this field empty if the state of the build should not depend on the number of warnings.
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