
Explanation/Reference:
Note:
Box 1 (see box 4): Empty elements are most often used to define SharePoint project items that lack a project or project item template in Visual Studio. When you add an empty element to your project, a node named EmptyElement[x] (where [x] is a unique number) is created. EmptyElement[x] contains a single file that is named Elements.xml. Use XML statements to define the desired elements in Elements.xml.
Box 2: SuiteLinksDelegate delegate control allow us to add links in right suite bar like "Newsfeed","Sites" etc.

Box 3: User Control Code Behind:
protected override void Render(HtmlTextWriter writer) etc.
Box 4: For all these delegate controls , you need to add a reference in Elements.xml file to specify the control id and control source properties:
<?xml version="1.0" encoding="utf-8"?>
<Elements xmlns="http://schemas.microsoft.com/sharepoint/">
<Control ControlSrc="/_controltemplates/15/MyDelegateControls/UserControl.ascx" Id="DelegateControlID" //ID of DelegateControl e.g. "SuiteLinksDelegate" Sequence="1" />
</Elements>
Reference: New Delegate Controls in SharePoint 2013