Which two aspects are applicable to Page Includes?

Two aspects that are applicable to PageIncludes are:

Standard Visualforce controls such as apex:form should not be used within a page include. This is because the page include is rendered inside an existing Visualforce page that already has a form element. Using another form element inside thepage include will cause conflicts and errors. Instead, the page include should use HTML elements or custom components that do not require a form element.

Page Includes can be configured as Body Includes Begin. This means that the page include will be rendered at the beginning of the body section of the page, before any other content or widgets. This can be useful for adding some custom content or functionality that applies to the whole page, such as a banner, a modal, or a script. Salesforce References: B2B Commerce and D2C Commerce Developer Guide, Page Includes