applyTodisplays the panel in the target element (the panel body can still contain any content), contentEluses the target element as its body content (the panel can still be transferred to any other element). These two methods are not related.
Edit : just notice that since version 4.0+ the setup applyTono longer exists. Over time, the declarative / progressive improvement strategy became less viable for Ext (it was never fully supported in all components), and they finally ceased to support it. You will always use it renderTo(for rendering programmatically to the target element), while it contentElis still valid and still indicates the element from which you can take content and add it to the panel body. contentElmost useful when you have some existing HTML on the page that you want to display inside the Panel component without having to reproduce the markup of the content in the JS code.
, , - ... , renderTo . items - renderTo , , .
, Ext renderTo, ( ) contentEl. contentEl AbstractComponent, HTML.