There are 3 kinds of page type within our theme: Page, Post, and Portfolio.
The Portfolio is a special post type, it only shows with the BM Content Builder plugin(installed/activated).
You could create the single portfolio items by the Portfolio type post(Portfolio > Add new). Check more
If you want to your Portfolios are shown(as items) on a portfolio list page (demo), the Category and Featured image are necessary. The Category is for organizing all the Portfolios. The Featured image is for the thumbnail in the portfolio list.
The Post is a default WordPress page type. It is usually for the single blog article. Check more
The Page is a default WordPress page type also. The Page is usually for creating the Front page, about, portfolio list, cart, checkout, and etc. Check more