The "Subpage - Default" template is the default layout for your subpages and will automatically be assigned when you create a new page. The layout displays fullwidth content, with the ability to display up to seven fullwidth sections below. 

The "Subpage - With Sidebar" template is an alternate option for your subpages and displays content down the right hand side. You can check out the "Sidebar" sections for some pre-designed monklet options.