The source code

    /*!
     * Ext JS Library 3.2.1
     * Copyright(c) 2006-2010 Ext JS, Inc.
     * licensing@extjs.com
     * http://www.extjs.com/license
     */
    /**
     * @class Ext.dd.DropTarget
     * @extends Ext.dd.DDTarget
     * A simple class that provides the basic implementation needed to make any element a drop target that can have
     * draggable items dropped onto it.  The drop has no effect until an implementation of notifyDrop is provided.
     * @constructor
     * @param {Mixed} el The container element
     * @param {Object} config
     */
    Ext.dd.DropTarget = function(el, config){
        this.el = Ext.get(el);

        Ext.apply(this, config);

        if(this.containerScroll){
            Ext.dd.ScrollManager.register(this.el);
        }

        Ext.dd.DropTarget.superclass.constructor.call(this, this.el.dom, this.ddGroup || this.group,
              {isTarget: true});

    };

    Ext.extend(Ext.dd.DropTarget, Ext.dd.DDTarget, {
        /**
         * @cfg {String} ddGroup
         * A named drag drop group to which this object belongs.  If a group is specified, then this object will only
         * interact with other drag drop objects in the same group (defaults to undefined).
         */
        /**
         * @cfg {String} overClass
         * The CSS class applied to the drop target element while the drag source is over it (defaults to "").
         */
        /**
         * @cfg {String} dropAllowed
         * The CSS class returned to the drag source when drop is allowed (defaults to "x-dd-drop-ok").
         */
        dropAllowed : "x-dd-drop-ok",
        /**
         * @cfg {String} dropNotAllowed
         * The CSS class returned to the drag source when drop is not allowed (defaults to "x-dd-drop-nodrop").
         */
        dropNotAllowed : "x-dd-drop-nodrop",

        // private
        isTarget : true,

        // private
        isNotifyTarget : true,

        /**
         * The function a {@link Ext.dd.DragSource} calls once to notify this drop target that the source is now over the
         * target.  This default implementation adds the CSS class specified by overClass (if any) to the drop element
         * and returns the dropAllowed config value.  This method should be overridden if drop validation is required.
         * @param {Ext.dd.DragSource} source The drag source that was dragged over this drop target
         * @param {Event} e The event
         * @param {Object} data An object containing arbitrary data supplied by the drag source
         * @return {String} status The CSS class that communicates the drop status back to the source so that the
         * underlying {@link Ext.dd.StatusProxy} can be updated
         */
        notifyEnter : function(dd, e, data){
            if(this.overClass){
                this.el.addClass(this.overClass);
            }
            return this.dropAllowed;
        },

        /**
         * The function a {@link Ext.dd.DragSource} calls continuously while it is being dragged over the target.
         * This method will be called on every mouse movement while the drag source is over the drop target.
         * This default implementation simply returns the dropAllowed config value.
         * @param {Ext.dd.DragSource} source The drag source that was dragged over this drop target
         * @param {Event} e The event
         * @param {Object} data An object containing arbitrary data supplied by the drag source
         * @return {String} status The CSS class that communicates the drop status back to the source so that the
         * underlying {@link Ext.dd.StatusProxy} can be updated
         */
        notifyOver : function(dd, e, data){
            return this.dropAllowed;
        },

        /**
         * The function a {@link Ext.dd.DragSource} calls once to notify this drop target that the source has been dragged
         * out of the target without dropping.  This default implementation simply removes the CSS class specified by
         * overClass (if any) from the drop element.
         * @param {Ext.dd.DragSource} source The drag source that was dragged over this drop target
         * @param {Event} e The event
         * @param {Object} data An object containing arbitrary data supplied by the drag source
         */
        notifyOut : function(dd, e, data){
            if(this.overClass){
                this.el.removeClass(this.overClass);
            }
        },

        /**
         * The function a {@link Ext.dd.DragSource} calls once to notify this drop target that the dragged item has
         * been dropped on it.  This method has no default implementation and returns false, so you must provide an
         * implementation that does something to process the drop event and returns true so that the drag source's
         * repair action does not run.
         * @param {Ext.dd.DragSource} source The drag source that was dragged over this drop target
         * @param {Event} e The event
         * @param {Object} data An object containing arbitrary data supplied by the drag source
         * @return {Boolean} True if the drop was valid, else false
         */
        notifyDrop : function(dd, e, data){
            return false;
        }
    });