code: protected ModifiableRule retrieveModifiableOverriden() {
              ModifiableRule overridenRule = null;
              //use local var to prevent potential concurent cleanup
              SipSession session = getSession();
              if (session != null && session.getServletContext() != null) {
                  String overridenRuleStr = session.getServletContext().getInitParameter(SYS_HDR_MOD_OVERRIDE);
                  if (overridenRuleStr != null)
                  {
                      overridenRule = ModifiableRule.valueOf(overridenRuleStr);
                  }
              }
              return overridenRule;
          }
docs: Allows to override the System header modifiable rule assignment.
  
  @return if the InitParameter is present at servletContext, or null
  otherwise