关于jQuery.ajax的一个小疑点

jQuery.ajax({
              type: 'POST',
              url: '/webs-resch/Research/SelectionConditionSettingAjax.do',
              data: {
                "searchColumnMode": searchColumnModeForAjax
              },
              beforeSend: function() {
                  handleBeforeSend1();
              },
              success: function(res) {   //这里的res是返回值？？从哪来的？
                handleHttpResponse(res)
              },
              error: function(res) {
                handleHttpErrorResponse()
              },
              timeout: 10000
            });