ext 问题求助

我想问的是,我的combo是从后台取数据来加载。root:"list",这个属性的意思我还是不太理解？
还有就是服务器返回的数据格式是？
就是xxAction.do方法返回什么样的数据。我拼接一个字符串然后发的页面？怎么发到页面，还有返回具体的格式和类型是什么。请帮我看看var store = new Ext.data.JsonStore({
           url:"xxAction.do",
           root:"list",
           fields:["name","type"],
           autoLoad:true
          });var win = new Ext.Window({
           id:"winnshan",
           title:"组合条件查询",
           modal:false,
           width:600,
           autoHeight:true,        
           items:new Ext.form.FormPanel({
                  autoHeight: true,
                  width   : 590,
                  bodyStyle: 'padding: 5px',
                  defaults: {
                      anchor: '0'
                  },
                  items: [{
                   layout:"column",
                      items:[{
                       id:"condition",
                       columnWidth:.25,
                       width:200,
                          store: store,
                          xtype:"combo",
                    displayField:'name',//显示值
                    valueField:"type",//实际值
                    typeAhead: true,
                    mode: 'remote',
                    forceSelection: true,//不允许用户自己输入值
                    triggerAction: 'all',
                    emptyText:'选择查询条件',
                    listeners:{'select':function(combo,record,index){
                         alert(Ext.getCmp("condition").getValue());
                    }}
                      }]
                  }],
                  buttons: []
              })
          });