code: private final void handleUserTxCallback(boolean txBegin)
      {
          // Obtain the extended-scoped persistence context binding accessor from
          // the JPA Service, which was registered by the EJB Container.     d515803
          JPAExPcBindingContextAccessor exPcAccessor = ivAbstractJPAComponent.getExPcBindingContext(); // F743-18776
  
          // This should never occur, but if it somehow does, then just disable
          // this callback so other transactions may continue working.       d515803
          if (exPcAccessor == null)
          {
              if (TraceComponent.isAnyTracingEnabled() && tc.isDebugEnabled())
                  Tr.debug(tc, "handleUserTxCallback : disabled - no Context Accessor");
              return;
          }
  
          // retrieve the caller's binding context from the thread
          JPAExPcBindingContext callerContext = exPcAccessor.getExPcBindingContext();
  
          if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled())
              Tr.entry(tc, "handleUserTxCallback : User Tx begin=" + txBegin + ", Caller binding context=\n\t--> "
                           + (callerContext != null ? callerContext.thisToString() : "{<<NONE>>}")); // d443713
  
          // (JPA 5.9.1 Container Responsibilities)
          // For stateful session beans with extended persistence contexts:
          // ** When a business method of the stateful session bean is invoked, if the stateful
          // session bean uses bean managed transaction demarcation and a UserTransaction is
          // begun within the method, the container associates the persistence context with
          // the JTA transaction and calls EntityManager.joinTransaction.
          if (callerContext != null &&
              callerContext.isBmt() &&
              callerContext.getExPcPuIds().length > 0)
          {
              callerContext.setBmtUserTxBegunInMethod(txBegin);
          }
          if (TraceComponent.isAnyTracingEnabled() && tc.isEntryEnabled())
              Tr.exit(tc, "handleUserTxCallback : " + callerContext);
      }
docs: When UserTransaction begin/end method is called, this method will mark this condition in
  the current component/business method invocation in the current SFSB binding context. This
  allows joinTransaction to be performed when any EntityManager interface method is called.