The default ruleset used by the Maven PMD Plugin, when no other ruleset is specified.
It contains the rules of the old (pre PMD 6.0.0) rulesets java-basic, java-empty, java-imports,
java-unnecessary, java-unusedcode.
This ruleset might be used as a starting point for an own customized ruleset [0].
[0] https://pmd.github.io/latest/pmd_userdocs_making_rulesets.html