Page
The fields of a library reside within a page. When a library is created, a single page exists, called MAIN, and it is the default page, so an Entry View card or Entry Edit card will show the fields of the MAIN page, when it opens.
The user may add pages and optionally set any one of them to be the default page. When there are multiple pages, the page names are set as tabs at the top of the card. The user may move between pages using those tabs, or the user may merely swipe the card left and right to change pages.
Each page primarily contains fields, but it may also contain one or more subheaders, which subdivide the fields on that page for visual organization.
Pages and subheaders are presentation structures and do not alter the data structure of the library.