Standalone Page

A Standalone Page is an Item that will not appear in the standard flow of Items of a collection.

Standalone pages will appear in the following situations:

Created by • Last edit by on 2019-10-21 01:44 • •

It will only appear when explicitly linked to, through a menu/button widget or a page list widget.

Not quite that clear :: see https://forums.b2evolution.net/sitewide-header-stand-alone-pages post#6
@fplanque points out that they are included in the site-wide header on the selected collection, which defaults to the home collection on a default install.
https://b2evolution.net/man/default-collections

they are included in the site-wide header

Because in b2evolution v6, the site-wide header includes a page list widget by default.