Page templates give concrete5 users a powerful way to create reusable page templates built using different designs and structures. Page Template Defaults extend this concept, allowing administrators to define "default" blocks to be automatically added to pages when they're created. If you know that pages of a certain type will always need to have a block of a certain type placed in a certain area of the page, Page Template Defaults is precisely the tool you'll want to use.
Even content itself can be pre-populated when a new page is created, saving editors and site contributors a lot of time that might otherwise be spent deploying duplicate content many times over. In practice, this can make setting up a complex page very easy, and allows an administrator to grant the privileges needed to create complext new pages to relatively nontechnical users.
From Pages & Themes > Page Types, we see a list of all page types currently installed on our site. Next to each, we see a button labeled "Output", among other buttons. This will proivde us with a list of compatible Page Templates for this page type. Clicking the "Edit Defaults" button next to one of the available templates, an editable "Default" version of a page template to be used as a master template of sorts for new pages created using that page type and template combination. Any blocks and settings we place on this page will also be placed on any new pages utilizing this same page type and template combo.
Already-existing pages that have this page type + template combo selected will not be affected, unless we use Set up on Child Pages (see below).
Set up on Child Pages
This option allows us to "push" content from the default page type template defaults to every page on the site that currently uses that page type and template.