code: private void checkNonCoreGroupId(Model model) throws EnforcerRuleException
      {
          String groupId = model.getGroupId();
          if (groupId.equals(CORE_GROUP_ID) || (groupId.startsWith(CORE_GROUP_ID_PREFIX)
              && !groupId.equals(CONTRIB_GROUP_ID) && !groupId.startsWith(CONTRIB_GROUP_ID_PREFIX))) {
              throw new EnforcerRuleException(
                  String.format("Contrib extension group id have to be prefixed with [%s]", CONTRIB_GROUP_ID));
          }
      }
docs: Make sure the group id does not start with "org.xwiki" or starts with "org.xwiki.contrib".