ContentSlots like Header and Footer are supposed to look the same on every Page.So they are defined for a PageTemplate. ContentSlots like the main content of a Page (e.g. the terms and conditions) may vary for each Page. So they are defined for a Page.
: The Web Component Slot element\n\n The HTML element part of the Web Components technology suite is a placeholder inside a web component that you can fill with your own markup, which lets you create separate DOM trees and present them together.