System blocks are pieces of default OpenCart functionality and are available only for the pages they originate from. E.g. the Products block is available only for pages that have product listing by default – these are categories or OpenCart layouts with
product/category route. The Subcategories block is available for the Category scope. The Breadcrumbs block is available for almost every page etc.
Some system blocks can be used multiple times on the same page and some only once.