You have three sub page templates to choose from.  The "Subpage Default" displays your page on a two column layout.  The "Subpage Default w/Linklist Nav" displays your page on a two column layout but the sidebar navigation is powered by a linklist.  The "Subpage Fullwidth" displays your page content on a full width layout.

Your template "Subpage Default" is your default page template and the "Style Guide" page has been setup to display your site default styles.  This template displays your sidebar navigation from the navigation tree.

Subpage Default w/Linklist Nav

When using this template you can use the field "Subnav Linklist" to enter a slug of a link list.  This link list will display as the sidebar navigation.  The slug is the name of the link list in lowercase with dashes in place of spaces eg. "My Sidebar Navigation" slug is "my-sidebar-navigation".