code: private void _saveRenderFlashMapTokenForNextRequest(FacesContext facesContext)
      {
          ExternalContext externalContext = facesContext.getExternalContext();
          String tokenValue = (String) externalContext.getRequestMap().get(FLASH_RENDER_MAP_TOKEN);
          ClientWindow clientWindow = externalContext.getClientWindow();
          if (clientWindow != null)
          {
              if (facesContext.getApplication().getStateManager().isSavingStateInClient(facesContext))
              {
                  //Use HttpSession or PortletSession object
                  Map<String, Object> sessionMap = externalContext.getSessionMap();
                  sessionMap.put(FLASH_RENDER_MAP_TOKEN+SEPARATOR_CHAR+clientWindow.getId(), tokenValue);
              }
              else
              {
                  FlashClientWindowTokenCollection lruMap = getFlashClientWindowTokenCollection(externalContext, true);
                  lruMap.put(clientWindow.getId(), tokenValue);
              }
          }
          else
          {
              HttpServletResponse httpResponse = ExternalContextUtils.getHttpServletResponse(externalContext);
              if (httpResponse != null)
              {
                  Cookie cookie = _createFlashCookie(FLASH_RENDER_MAP_TOKEN, tokenValue, externalContext);
                  httpResponse.addCookie(cookie);
              }
              else
              {
                  //Use HttpSession or PortletSession object
                  Map<String, Object> sessionMap = externalContext.getSessionMap();
                  sessionMap.put(FLASH_RENDER_MAP_TOKEN, tokenValue);
              }
          }
      }
docs: Take the render map key and store it as a key for the next request.
  
  On the next request we can get it with _getRenderFlashMapTokenFromPreviousRequest().
  @param externalContext